Add source code location for generators into heap snapshot file.

Bug: chromium:854097
Change-Id: I726b245a707515502976476703e57b7f58c92782
Reviewed-on: https://chromium-review.googlesource.com/1174433
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55254}