This fixes collection of function decoding time (See comment #20 of
v8:6361). Updated other path for decoding to also update UMA stat.

Bug: v8:6361
Change-Id: I8e6cb398aebd7847e064d3b9a979586ded118513
Reviewed-on: https://chromium-review.googlesource.com/598458
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47136}