Make |Heap::findGCInfo()| threadsafe.

Before this patch, |Heap::findGCInfo()|'s iteration on
AttachedThreadStateSet was not guarded. This triggered "modification
while iteration" assert when |Heap::findGCInfo()| was called from
non-safepoint.

This patch exposes |ThreadState::threadAttachMutex()| and
|Heap::findGCInfo()| will acquire lock on it when we are not in safepoint.

Unfortunately there are no trybot which has an ENABLE(GC_TRACING) flag.
NOTRY=true
TEST=content_shell --dump-render-tree http/tests/workers/terminate-during-sync-operation-websql.html wont crash.
BUG=380474

Review URL: https://codereview.chromium.org/311263008

git-svn-id: svn://svn.chromium.org/blink/trunk@175774 bbb929c8-8fbe-4397-9dbb-9b2b20218538