Set the WebViewPlugin::WebViewHelper as the WebWidgetClient.

WebViewHelper is a WebWidgetClient even though WebViewPlugin's WebView
is not composited. So set it up as the WebWidgetClient after attaching
the main frame.

R=dcheng@chromium.org, tommycli@chromium.org

Bug: 920952, 912193
Change-Id: I8391bde1d0452cf78d23a8b855a45497bac02d66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504142
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638497}
3 files changed