Ensure that RuntimeCallStats::Enter is paired with Leave when
FLAG_runtime_stats changes in flight.

BUG=chromium:669329

Change-Id: I4da7edf88990fdebd7d05325a09cfca0702cfe5a
Reviewed-on: https://chromium-review.googlesource.com/643472Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47749}