Rename WebViewImpl::focusedWebCoreNode to focusedElement, and it should return Element*.

- 'Node' in the name should be 'Element'.
  Only elements are focusable.
- We remove 'WebCore' in the name.
  It adds no information. Element usually means WebCore::Element.

Also, wrap Document::focusedNode callsites with toElement because
Document::focusedNode alwyas reutnrs an Element.

This is a preparation to rename Document::focusedNode to focusedElement.

BUG=258763

Review URL: https://chromiumcodereview.appspot.com/19211003

git-svn-id: svn://svn.chromium.org/blink/trunk@154336 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add callouts to WebTestProxy for methods moving from RenderViewImpl to RenderFrameImpl.

The new codepath will be activated once the Chromium side of the changes lands. The current Chromium CL is https://codereview.chromium.org/16418003.

BUG=245126

Review URL: https://chromiumcodereview.appspot.com/19266002

git-svn-id: svn://svn.chromium.org/blink/trunk@154337 bbb929c8-8fbe-4397-9dbb-9b2b20218538