Fix a crash in the renderer process which occurs in the PPAPI host code due to the underlying plugin

Fix a crash in the renderer process which occurs in the PPAPI host code due to the underlying plugin
object getting destroyed in the context of an incoming ExecuteScript call while the renderer is waiting
for an javascript function call on the plugin object to return.

We need to grab a reference on the plugin module in the dispatcher code and in the plugin object as
there are two crashes which occur here

1. When the dispatcher send is about to return from the Send call and it tries to unmarshal the return
   values.
2. The other crash is in the wrapper class Invoke function where we crash similarly.

Fixes bug http://code.google.com/p/flapper/issues/detail?id=77
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=103570

[piman, 2011-09-30 22:23:12]

LGTM