Oilpan: support OSX thread stack size discovery.

We do know enough about OSX stack sizes to be able to work around
bugginess of 10.9's pthread_get_stacksize_np().

R=haraken
BUG=569480

Review URL: https://codereview.chromium.org/1527513002

Cr-Commit-Position: refs/heads/master@{#364996}
1 file changed