The stack can be cleared with RuntimeCallStats::Reset() call.
Correctly handle the case by silently exit the running timer scopes.

BUG=chromium:760649

Change-Id: I51ecca5591a7af358f3e50779d0f81cb9d76e502
Reviewed-on: https://chromium-review.googlesource.com/734121Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48847}