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Aura requires that display::Screen::GetScreen() returns a valid object.
1. Added WebRunnerContentBrowserClient,
2. Added WebRunnerBrowserMainParts,
3. Added display::Screen, created by WebRunnerBrowserMainParts.

Bug: 822474
Change-Id: Id22a6a369b89ed0f911513adb17f33ed8b503f26
Reviewed-on: https://chromium-review.googlesource.com/988841
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547481}