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With the removal of the RenderWidgetFullscreenPepper WebExternalWidget
is only used inside tests. Move these tests inside blink.
Add ability to override the WebViewFrameWidget with a custom implementation
so that we can instrument it for tests.

BUG=1097816

Change-Id: Id5cf6cea5d56f91aa6d7c0c5c05ff98f7ec9e19a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2460715
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815323}