The key -> value edge is shown as "<index> / WeakMap", where <index> is
the index of the edge in the key.

Bug: chromium:778739, chromium:749502
Change-Id: I657051695f2a171372788dbb777543a55a35d554
Reviewed-on: https://chromium-review.googlesource.com/926524
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51559}