Add mutex when accessing concurrent recompilation output queue.

Add mutex when accessing concurrent recompilation output queue.

Also small refactorings.

R=mstarzinger@chromium.org
BUG=291236
Committed: https://code.google.com/p/v8/source/detail?r=16740

[Michael Starzinger, 2013-09-16 15:28:19]

LGTM if comment about the stack guard is addressed.

https://codereview.chromium.org/23902033/diff/1/src/optimizing-compiler-threa...
File src/optimizing-compiler-thread.cc (right):

https://codereview.chromium.org/23902033/diff/1/src/optimizing-compiler-threa...
src/optimizing-compiler-thread.cc:140: osr_candidates_.Clear();
As discussed offline: It is really scary that "osr_candidates_" is a simple
list, but it's only a temporary solution.

https://codereview.chromium.org/23902033/diff/1/src/optimizing-compiler-threa...
src/optimizing-compiler-thread.cc:206:
isolate_->stack_guard()->Continue(INSTALL_CODE);
Please move this back into Execution::HandleStackGuardInterrupt, this has
nothing to do with the compiler thread, it is specific to how we handle stack
guards.

[Yang, 2013-09-16 15:34:55]

Committed patchset #2 manually as r16740 (presubmit successful).