Chrome no longer calls v8::Isolate::Dispose on shutdown, essentially preventing
the use of V8 stats within chrome/content_shell. This CL adds a basic hook to
the api that is then used to only print the stats.

Chrome change: https://codereview.chromium.org/2693353002

Change-Id: I1481c14afe611e9c08ae67c815201a45940daa57
Reviewed-on: https://chromium-review.googlesource.com/452338Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43706}