Cleanup various WebView/WebFrame APIs to properly handle remote frames.

WebCore can now also properly set a RemoteFrame as the main frame.

BUG=346764

Review URL: https://codereview.chromium.org/338993003

git-svn-id: svn://svn.chromium.org/blink/trunk@176551 bbb929c8-8fbe-4397-9dbb-9b2b20218538