Import ConditionVariable class.

Import ConditionVariable class.

Condition variables are synchronization primitives that can be used
to block one or more threads while waiting for condition to become
true.

Right now we have only semaphores, mutexes and atomic operations for
synchronization, which results in quite complex solutions where an
implementation using condition variables and mutexes would be straight
forward.

There's also a performance benefit to condition variables and mutexes
vs semaphores, especially on Windows, where semaphores are kernel
objects, while mutexes are implemented as fast critical sections,
it CAN be beneficial performance-wise to use condition variables
instead of semaphores.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23548007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16492 ce2b1a6d-e550-0410-aec6-3dcde31c8c00