This CL adds an uma stat to track the throughput
of lazy compilation for wasm functions in KB/s.

BUG=chromium:770618
R=kschimpf@chromium.org,bradnelson@chromium.org,isherman@chromium.org

Change-Id: Iad06cfb1f185f7e2ab6b0198282c03c3d8f29e2e
Reviewed-on: https://chromium-review.googlesource.com/706276
Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48475}