Remove preferredExtensionForMIMEType from Blink.

This will be followed by a Chromium change to remove the implementations
of this.

BUG=268948

Review URL: https://chromiumcodereview.appspot.com/22692003

git-svn-id: svn://svn.chromium.org/blink/trunk@155894 bbb929c8-8fbe-4397-9dbb-9b2b20218538