[PPAPI] ResourceVar now reference counts its Resource in the plugin.

ResourceVar is now an abstract base class with subclasses
HostResourceVar and PluginResourceVar. The PluginResourceVar has a
reference counted Resource instead of a PP_Resource.

VarTracker has MakeResourceVar and MakeResourcePPVar methods, to
abstract over the creation of a resource var of the correct subclass.

Also, the creation_message is now NULL when empty, instead of being an
empty message object.

BUG=290713

Review URL: https://chromiumcodereview.appspot.com/23809016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224717 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224717