[PPAPI] V8VarConverter now calls ResourceConverter to create resources.

Currently, there are no resources that ResourceConverter creates, so
this is just a preliminary CL to set up the necessary infrastructure.

Also added some ancillary functionality to ResourceConverter:
- Implemented CreateResourceVar, it now creates a host resource var.
- FlushComplete now takes the completed pending browser host IDs and
  stores them in the resource vars.

BUG=177017

Review URL: https://codereview.chromium.org/24507002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225956 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225956