Bug 641685 - Make plugin object map a singleton in the plugin process (r=bsmedberg)

This patches moves the object map (which tracks the PluginScriptableObjectChild
and PluginInstanceChild for a given NPObject) from the PluginModuleChild to
a global variable. This change prepares the way for having multiple PluginModuleChild
instances in a given plugin process.
Bug 641685 - [e10s] Make setTestPluginEnabledState work in content processes (r=bsmedberg)
Bug 1085710 - Stop trying to send nsIPluginTag over IPC (r=mconley)
Bug 641685 - Make sure generated code for IPDL bridged includes all necessary forward decls (r=bsmedberg)
Bug 641685 - Start plugins from the chrome process in e10s (r=bsmedberg)

This patch has a few side effects:
1. Plugins in the chrome process are "mirrored" to all content processes,
although this mirroring is currently imperfect (bug 1090576)
2. Plugins are no longer sorted by modification date in nsPluginHost.
3. Plugin exceptions are no longer propagated to JS code. They are ignored.