Before this patch the embedder could assign timestamp to the last interval after calling GetHeapStats. This would be slightly different from the timstamps assigned by v8 internally and written into heap snapshot. This patch allow to avoid this small discrepancy by returning timestamp along with last heap stats update.

BUG=chromium:467222
LOG=Y

Review URL: https://codereview.chromium.org/1037803002

Cr-Commit-Position: refs/heads/master@{#27466}