Fix layering violations in WebFrameWidget.

There are currently Blink-internal interfaces exposed on the public
WebFrameWidget interface. Move these to an internal interface: once
WebWidget is properly separated from WebView, WebViewFrameWidget will be
removed and WebFrameWidgetImpl and WebFrameWidgetBase can be merged.

BUG=419087

Review-Url: https://codereview.chromium.org/2160003003
Cr-Commit-Position: refs/heads/master@{#406254}
15 files changed