Oilpan: Fix Heap::isConsistentForGC which only checks the heap for one attached thread at this point.

Thanks for noticing this Peter. :)

R=oilpan-reviews@chromium.org, pkasting@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/206113002

git-svn-id: svn://svn.chromium.org/blink/trunk@169635 bbb929c8-8fbe-4397-9dbb-9b2b20218538