Remove unused WebView::setFixedLayoutSize method.

All use of this method in Chrome has been removed, the code can now
be dropped.

BUG=417221

Review URL: https://codereview.chromium.org/597143002

git-svn-id: svn://svn.chromium.org/blink/trunk@182666 bbb929c8-8fbe-4397-9dbb-9b2b20218538