Add a counter to measure the pure liftoff compile time. With background
compilation, the reported result will roughly match the cpu time, not
wall-clock time.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I9c76673a4cbe833de5b41a23d09892f577401374
Reviewed-on: https://chromium-review.googlesource.com/853859Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50412}