Chromium does not throw the exception message by calling NPN_SetException from NPAPI plugin

Fix the issue that chromium does not throw expected Javascript exception
when an NPAPI plugin calls NPN_SetException() as described on 
http://code.google.com/p/chromium/issues/detail?id=68919.

This fixing depends on the patch from WebKit bugzilla for
https://bugs.webkit.org/show_bug.cgi?id=72484.

The root cause is that the exception set by WebBindings::setException in 
PluginChannelHost::OnSetException will be overwritten by WebCore::npObjectInvokeImpl
in case of the return value of excuting invokation on NPObject is false.

This patch introduces a new IPC message NPObjectMsg_SetException for
NPObjectProxy. If NPN_SetException is called from plugin process, it
will handle it to remember an exception has been thrown and tell V8 engine
to not throw a general exception again via a returned result parameter.

NPObjectProxy::OnSetException is required to be only called in renderer 
process because it is meaningless to throw an Javascript exception in
plugin process.

BUG=chromium:68919
TEST=npobject_set_exception.html

[Hongbo, 2011-11-24 05:24:08]

Could you pls review this patch? Thanks