The code was updating the UMA stats for V8.WasmFunctionSizeBytes when
the WASM function was decoded. Unfortunately, decoding of functions is
no longer done as a separate step. Rather, it is done as part of the
compilation step.

This CL moves the UMA updates to the compilation step.

Bug: v8:7032
Change-Id: I4679036035540fabd43855c1ba5ba66ffee6762a
Reviewed-on: https://chromium-review.googlesource.com/754023
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49117}