Oilpan: catch some self-referential leaks (main thread.)

Track the number of SelfKeepAlive<>s that are currently active on
the main thread, so as to be able to detect if any remains on
shutdown. A clear indication of a leak, if so.

Also add some debugging support for detecting thread shutdown
leaks due to Persistent<>s not being released; see
PersistentRegion::dumpLivePersistents() and ThreadState::cleanup().

R=haraken
BUG=420515

Review URL: https://codereview.chromium.org/1275863002

git-svn-id: svn://svn.chromium.org/blink/trunk@200174 bbb929c8-8fbe-4397-9dbb-9b2b20218538
5 files changed