Check for these UMA stats were in functions only used for testing. Moved the
checks to code inside module decoding.

Note that the module decoder is used both to validate and generate the
intermediate (turbofan) graph of function bodies.  This CL assumes
that the validation phase (of function bodies) is the correct place to
track decoding time.

Bug: v8:6361
Change-Id: I791281daae96473d53cb8ae332fff8bb2673bf6c
Reviewed-on: https://chromium-review.googlesource.com/586974
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46915}