The sampling heap profiles can now be retrieved without stopping
the profiler.

BUG=v8:6887

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I491206d0bafd7d4e198622117c12aab0057e6bc6
Reviewed-on: https://chromium-review.googlesource.com/749700Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49160}