Currently a WeakMap key edge shows up as '<id> / WeakMap'. This patch
changes it to '<id> / key <KeyName> in WeakMap'.

Bug: chromium:827713
Change-Id: I0306bacdc331ab69739be88cf29d16a21187625f
Reviewed-on: https://chromium-review.googlesource.com/992035Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52465}