https://chromium-review.googlesource.com/530193 accidentally disabled
the timer for compilation time of individual wasm functions. This CL
reenables this, and also gets rid of the ExecuteCompilationInternal
method by using base::Optional for conditionally initializing the
HistogramTimerScope.

R=mtrofin@chromium.org
CC=ahaas@chromium.org

Change-Id: I359f0622561b82aea4d4002b7ed79fde32f1b35e
Reviewed-on: https://chromium-review.googlesource.com/544135Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46147}