Add WebDocument::clearFocusedElement

WebDocument::clearFocusedNode should be clearFocusedElement.

BUG=258763

Review URL: https://codereview.chromium.org/179013002

git-svn-id: svn://svn.chromium.org/blink/trunk@167738 bbb929c8-8fbe-4397-9dbb-9b2b20218538