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This flag will be used to build out support for scoping FullscreenController
to a Browser while keeping the current BrowserState-scoped logic as default.

Bug: 790886
Change-Id: Iac1811181a6313271561ffecae306fe4b7b94684
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007314
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734534}