Make PP_Resources associated with the Instance rather than the module. This

adds PP_Instance to the necessary places in the API to make this possible.

String and Object vars used to be PP_Resources. But it is not practical to
assocaited strings with an instance since then we can't have implicit var
constructors and have to litter every string with an instance. So this changes
vars to use their own tracking system associated with the module (i.e. keeping
the current semantics) and making it no longer a resource. I made the internal
Var IDs 32 bits since Neb is about to land his 64->32 change.

Now it force-deletes resources associated with an instance when that instance
goes away. I added some additional code and tracking in ResourceTracker to do
this. I could then remove the Instance::Observer class since the resource can
use the (now renamed) StoppedTracking to know that it's being deleted in
response to the instance being destroyed.

TEST=ppapi ui tests
BUG=none

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@71544 0039d316-1c4b-4281-b951-d872f2087c98