Make refcounting of IndexedDBFactory threadsafe

Make refcounting of IndexedDBFactory threadsafe

When IndexedDBContext is destroyed it posts a task to the IDB thread to
run IndexedDBFactory::ContextDestroyed() to do some cleanup, but drops
its own reference, leading to a refcounting/threading race. Follow the
rules in base/callbacks.h to use RefCountedThreadSafe<> to fix.

R=alecflett,dgrogan
TBR=glider
BUG=321596
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=236842

[jsbell, 2013-11-20 20:02:58]

alecflett@, dgrogan@, glider@ - please take a look?

I'm having trouble reproducing w/ tsan (always times out), so this is a
speculative fix.

[alecflett, 2013-11-22 18:13:39]

lgtm

[commit-bot: I haz the power, 2013-11-22 18:15:08]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jsbell@chromium.org/77863005/30001

[commit-bot: I haz the power, 2013-11-22 18:37:57]

Retried try job too often on chromium_presubmit for step(s) presubmit
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=chromium_p...

[commit-bot: I haz the power, 2013-11-22 19:48:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jsbell@chromium.org/77863005/30001

[commit-bot: I haz the power, 2013-11-22 22:21:11]

Change committed as 236842