CPDOMAvailable was read during load before it was assigned.
* CPCompatibility: Move the CPDOMAvailable assignment ahead of the engine detection, whose AppleWebKit case calls +[CPPlatform isBrowser], which reads it.
* CPCompatibilityTest: Add a test that re-executes CPCompatibility.j with an AppleWebKit user agent and CPDOMAvailable unset, failing on any exception.
