This fixes HeapSnapshotGenerator::SetProgressTotal so that
ProgressReport is called with finished flag only once.

The DevTools front-end assumes that progress with finished flag is
reported only once.

Change-Id: Iad958478aa8ad27a520cb491419e521027967754
Reviewed-on: https://chromium-review.googlesource.com/949224
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51767}