FutexEmulation::Wait can potentially block forever on a condition variable. We
want to allow this to be interrupted (for a debugger, or to terminate the
thread, for example).

The previous implementation would periodically wake up the waiter to check for
interrupts. This CL modifies the StackGuard so it wakes the blocked futex if
the thread should be interrupted.

BUG=chromium:497295
R=jarin@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1230303005

Cr-Commit-Position: refs/heads/master@{#30311}