Implement screenshot comparison

This change added the implementation of screenshot comparison with
ChromeWebDriver which will be used in the open-source code base.

This change also set a default window size(1200x2000) for each screenshot
test. This is to make the size of screenshot deterministic to help
build the screenshot comparison tests.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=235539713

javatests/google/registry/webdriver/ActualScreenshot.java

@@ -0,0 +1,94 @@
+// Copyright 2019 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.webdriver;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Preconditions.checkState;
+
+import java.awt.image.BufferedImage;
+import java.io.ByteArrayInputStream;
+import java.io.File;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.Arrays;
+import javax.imageio.ImageIO;
+import javax.imageio.ImageReader;
+
+/** An immutable class represents . */
+public class ActualScreenshot {
+  public static final String IMAGE_FORMAT = "png";
+  private String imageKey;
+  private BufferedImage bufferedImage;
+
+  private ActualScreenshot(String imageKey, BufferedImage bufferedImage) {
+    this.imageKey = imageKey;
+    this.bufferedImage = bufferedImage;
+  }
+
+  /** Creates an ActualScreenshot from the given image format and byte array. */
+  public static ActualScreenshot create(String imageKey, byte[] imageBytes) {
+    checkNotNull(imageKey);
+    checkNotNull(imageBytes);
+    byte[] imageBytesClone = Arrays.copyOf(imageBytes, imageBytes.length);
+    ByteArrayInputStream imageInputStream = new ByteArrayInputStream(imageBytesClone);
+    ImageReader imageReader = ImageIO.getImageReadersByFormatName(IMAGE_FORMAT).next();
+    try {
+      imageReader.setInput(checkNotNull(ImageIO.createImageInputStream(imageInputStream)));
+      BufferedImage bufferedImage = checkNotNull(imageReader.read(0));
+      return new ActualScreenshot(imageKey, bufferedImage);
+    } catch (IOException e) {
+      throw new UncheckedIOException(e);
+    }
+  }
+
+  /** {@link BufferedImage#getSubimage(int, int, int, int)} */
+  public ActualScreenshot getSubimage(int x, int y, int w, int h) {
+    return new ActualScreenshot(imageKey, bufferedImage.getSubimage(x, y, w, h));
+  }
+
+  /** {@link BufferedImage#getWidth()} */
+  public int getWidth() {
+    return bufferedImage.getWidth();
+  }
+
+  /** {@link BufferedImage#getHeight()} */
+  public int getHeight() {
+    return bufferedImage.getHeight();
+  }
+
+  /** {@link BufferedImage#getRGB(int, int)} */
+  public int getRGB(int x, int y) {
+    return bufferedImage.getRGB(x, y);
+  }
+
+  /** Writes the underlying BufferedImage to the given file. */
+  public void writeTo(File file) {
+    try {
+      checkState(ImageIO.write(bufferedImage, IMAGE_FORMAT, checkNotNull(file)));
+    } catch (IOException e) {
+      throw new UncheckedIOException(e);
+    }
+  }
+
+  /** Returns the imageKey of the screenshot. */
+  public String getImageKey() {
+    return imageKey;
+  }
+
+  /** Returns the concat of imageKey and imageFormat. */
+  public String getImageName() {
+    return String.join(".", imageKey, IMAGE_FORMAT);
+  }
+}