Prevent reentrant destructors in the PluginModule.

Prevent reentrant destructors in the PluginModule.

This makes us not hold a ref to the PluginModule during sending async messages.
Previously we would cancel callbacks in the PluginModule destructor and this
would change the ref count from 0 to 1, and then back to 0, calling reentrantly
into the destructor again.

Not holding a ref during async messages prevent this problem. I also added
some checking to detect sync messages during shutdown, and also reentrant
destructors should this happen again.

BUG = http://crosbug.com/21258, http://crbug.com/99398
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=104847

[viettrungluu, 2011-10-10 23:39:17]

LGTM.