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Adds ChromeDriver components necessary to leverage DevTools
Target.createTarget changes, allowing for new window or tab to be
be created based on user preference in both headless and regular Chrome.
This completes the implementation of the NewWindow command from the
W3C-spec (https://w3c.github.io/webdriver/#new-window).

Design Doc: http://bit.ly/chromedriver-new-window

focus/blur test to confirm that focussing steps are not run during
creation.

Tested: WPTs webdriver/tests/new_window, all run_py_tests, and added
Bug: chromedriver:2690, b/127692156
Change-Id: I638167a47aa759ce511aae734c615cc6cf76df59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636464
Commit-Queue: Rohan Pavone <rohpavone@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665612}