[PPAPI] Added serialization for resource PP_Vars.

This adds a RawVarData subclass, ResourceRawVarData, which allows vars with type
PP_VARTYPE_RESOURCE to be serialized and deserialized.

BUG=177017

Review URL: https://chromiumcodereview.appspot.com/23769010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224969 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224969