If there is a WeakMap in a retaining path of an object, then we have
two choices:
1) use the WeakMap backing store as the retainer.
2) use the key in the WeakMap as the retainer.
Both cases can be useful for debugging memory leaks.

This patch adds a second parameter to %DebugTrackRetainingPath().

If this parameter is "track-ephemeral-path", then the retaining path
printer will use the key in the WeakMap as a retainer. Otherwise,
the WeakMap backing store will be reported as the retainer.

Bug: v8:6987
Change-Id: I9feae25bc734c3abed501b9f901a36d2248a105c
Reviewed-on: https://chromium-review.googlesource.com/753343
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49324}