Remove RuntimeEnableFeature flag for RepaintAfterLayout.

The non-RepaintAfterLayout code path has been removed and the chromium
command line flags cleaned up. This CL removes the runtime enabled feature
for RepaintAfterLayout which is the last piece of the cleanup.

BUG=385281

Review URL: https://codereview.chromium.org/350343005

git-svn-id: svn://svn.chromium.org/blink/trunk@176939 bbb929c8-8fbe-4397-9dbb-9b2b20218538