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This change makes all WebWidgets be created by blink. Since all
WebWidgets are created by blink they have a common base member
WidgetBase which will eventually create and own
the compositing, input and emulation infrastructure.

Change-Id: I306658e8d3b941bcbb93f864e503112387dbd169
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042510Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746841}