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
This change does a few things:
1. Partially externalized WebDriver tests by using ChromeDriver
as an implementation of WebDriver API in the external build.
2. Refactored WebDriverRule.java to decouple the creation and
using of WebDriver related stuff so we can have different
implementations in internal and external builds.
3. Refactored the usage of some internal libraries to have a
central place to store all of them to make it easier to
remove them in the external build.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=233661757
