[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

• content/content_renderer.gypi[diff]
• content/renderer/pepper/host_resource_var.cc[Added - diff]
• content/renderer/pepper/host_resource_var.h[Added - diff]
• content/renderer/pepper/host_var_tracker.cc[diff]
• content/renderer/pepper/host_var_tracker.h[diff]
• ppapi/ppapi_proxy.gypi[diff]
• ppapi/proxy/plugin_resource_var.cc[Added - diff]
• ppapi/proxy/plugin_resource_var.h[Added - diff]
• ppapi/proxy/plugin_var_tracker.cc[diff]
• ppapi/proxy/plugin_var_tracker.h[diff]
• ppapi/shared_impl/resource_var.cc[diff]
• ppapi/shared_impl/resource_var.h[diff]
• ppapi/shared_impl/test_globals.h[diff]
• ppapi/shared_impl/unittest_utils.cc[diff]
• ppapi/shared_impl/var.cc[diff]
• ppapi/shared_impl/var_tracker.cc[diff]
• ppapi/shared_impl/var_tracker.h[diff]