Removes from CL https://codereview.chromium.org/2929853003 code to fix
histogram timers in class WasmCompilationUnit. This was done because
the CL was reverted due to errors caused by background compiles that
updated UMA histogram timers.

The goal of this CL is to reland the remaining portion of the reverted
CL.

Bug:v8:6361

Change-Id: Ic03ceb118734bd55c463a843521bcd5b09342afe
Reviewed-on: https://chromium-review.googlesource.com/550196Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Karl Schimpf <kschimpf@google.com>
Cr-Commit-Position: refs/heads/master@{#46268}