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