V8 tracing controller uses 2 clocks: wall clock and cpu clock. This patch
implements CPU time for OS X and POSIX to provide more accurate
accounting of CPU time used by each thread.

BUG=v8:4984
LOG=n

Review-Url: https://codereview.chromium.org/1959103004
Cr-Commit-Position: refs/heads/master@{#36188}