Experimental scanner multithreading fix.

Experimental scanner multithreading fix.

Many ExperimentalScanners might get created simultaneously in different threads,
so we cannot just keep track of them without locking. Moving the bookkeeping
into Isolate.

R=ulan@chromium.org
BUG=

Committed: https://code.google.com/p/v8/source/detail?r=18199

[ulan, 2013-12-02 15:05:42]

LGTM

https://codereview.chromium.org/99243002/diff/1/src/isolate.cc
File src/isolate.cc (right):

https://codereview.chromium.org/99243002/diff/1/src/isolate.cc#newcode2543
src/isolate.cc:2543: heap()->AddGCEpilogueCallback(
Not sure. It is probably better do AddGCEpilogueCallback at initialization once.
Because adding/removing callbacks is linear in number of existing callbacks. In
the worst case we will do this on each compilation.

[marja, 2013-12-02 15:07:30]

Committed patchset #2 manually as r18199 (presubmit successful).