This bug involves three steps:
1. Move WebCore's threading primitives to wtf.
2. Adjust WebCore's dependencies to find threading inside wtf.
Consider the internal threading changes made for Bug 4635 -- they might be valid for win32.
Also consider the internal threading changes made for Bug 4645 -- they might be
valid for win32.
Resolved by changes landed in Changeset 30522 (and subsequent fixes).