Remove WebPopupMenuImpl::enterForceCompositingMode

Create the compositor in WebPopupMenuImpl::initialize because it's always going to be used.

BUG=378029

Review URL: https://codereview.chromium.org/304273002

git-svn-id: svn://svn.chromium.org/blink/trunk@175160 bbb929c8-8fbe-4397-9dbb-9b2b20218538