AggregatableHistogramTimer will always add a sample when it is destroyed,
even if there were no AggregatedHistogramTimerScope called within it. This
makes the V8.CompileLazyMicroSeconds histogram not particularly useful since
it is 90% filled with zero entries where v8 execute didn't require any
lazy function compilation

Change-Id: Ia75c8596237b22528cbba3e8ae2b67e28ea54097
Reviewed-on: https://chromium-review.googlesource.com/793452Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49674}