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Added GetRootWindowForNewWindows and implemented in wm_helper_chromeos.
This allows simple retrieval of root window from
window manager, used for setting parent window when creating
widgets (https://chromium-review.googlesource.com/c/chromium/src/+/1298172/20,
fullscreen_shell_surface.cc).

Bug: 896710
Test: Built and ran on ChromeOS
Change-Id: Ie43bea17c31cdf79402fadea441d5bc924e64503
Reviewed-on: https://chromium-review.googlesource.com/c/1302823Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#605806}