Use expected main frame size from WebViewImpl when creating a new FrameView.

The main frame's size is no longer simply the viewport size. Instead,
WebViewImpl sizes it after layout to be the either the content width with
a height matching the viewport aspect ratio, or the viewport at minimum page
scale. This fixes a bug where the innerWidth/innerHeight values would be
wrong during an onload event if the onload would happen before the content was
properly sized.

BUG=431097,428722

Review URL: https://codereview.chromium.org/705423002

git-svn-id: svn://svn.chromium.org/blink/trunk@185145 bbb929c8-8fbe-4397-9dbb-9b2b20218538