Bug 1035454, part 1 - Increase the leak threshold for tab processes. r=khuey
Bug 1035454, part 2 - Paper over late shutdown failures on B2G and Windows in debug content processes by exiting XPCOM early. r=bsmedberg

On B2G, there are crashes very late in shutdown on content processes. On Windows XP,
there is an intermittent test failure. We work around both of these by calling exit(0)
during XPCOM shutdown prior to the points where these errors occur. This enables us to
land part 4, that stops us from crashing in content processes when the xpcom-shutdown
message is sent, and enables leak checking in content processes on Linux.
Bug 1035454, part 3 - Disable test_bug345339.html on e10s for leaking. r=jst
Bug 1035454, part 4 - Don't send 'Webapps:UnregisterForMessages' at xpcom-shutdown, r=fabrice.