That patch introduces EmbedderGraph interface that embedders can use to
represent C++ objects that retain or are retained by V8 JS objects.

The heap snapshot generator adds nodes and edges of the EmbedderGraph to
the heap snapshot, allowing arbitrarily complex retaining paths that
cross V8/Embedder boundary.

The new functionality is enabled only if the embedder sets the
BuildEmbedderGraph callback.


Bug: chromium:749490

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I10a1fa000d6d4ba47fc19d84c7cfc2c619d496fc
Reviewed-on: https://chromium-review.googlesource.com/890521Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51016}