Add names for fields in JSGeneratorObjects in Heap Snapshot
Generator.

Bug: chromium:854097
Change-Id: I075acf0821c9d002535b4fdc4ce4ddbb2fc9627c
Reviewed-on: https://chromium-review.googlesource.com/1148387
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54652}