[ChromeDriver] Implementation of Get Window Rect

This command is required for running some WPTs for WebDriver.
Will add integration tests in a follow up CL implementing Set Window Rect

spec:https://w3c.github.io/webdriver/webdriver-spec.html#get-window-rect


Bug: chromedriver:1940
Change-Id: Idef3513dc5ad26fe8dd82d9808e32e84e8f7ad80
Reviewed-on: https://chromium-review.googlesource.com/685639
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Jonathon Kereliuk <kereliuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506432}
5 files changed