Remove WebCore::invalidPlatformFileHandle and WebCore::isHandleValid.

These are unused, so we can get rid of these and the corresponding FIXME.

BUG=265858

Review URL: https://chromiumcodereview.appspot.com/21131006

git-svn-id: svn://svn.chromium.org/blink/trunk@155276 bbb929c8-8fbe-4397-9dbb-9b2b20218538