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Apart from start making use of the always-true downcast
helper, in order to ensure that WebFrameWidget is always a
WebFrameWidgetBase intance (and hence respecting the downcast
helper contract), this CL also adds a private WebFrameWidget
ctor, and make it a friend-class to WebFrameWidgetBase.

This ensures that WebFrameWidgetBase is the only concrete subclass
that implements WebFrameWidget.

R=jbroman@chromium.org
CC=blink-reviews-vendor@chromium.org

BUG=891908

Change-Id: Ibdea5aea0fcb538b15a12a7cfc00691249ef2099
Reviewed-on: https://chromium-review.googlesource.com/c/1490291
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636234}