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There is no need that this API needs to exist on the FrameWidget,
we can go directly to the the WebView from WebLocalFrameImpl when
notifying of detaching the frame tree.

BUG=1097816

Change-Id: Ida6fada222c2dd972d6f4661c8c8c6680f81205d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532668Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826484}