GitLab

This allows the WorkerWatcher to cope with the case where there are
multiple Frame<->Worker relationships in existence at the same time
for a single Frame, Worker pair. Multiple relationships are
represented as a single edge in the graph, which is added/removed
on 0<->1 count transitions.

Bug: 1143281
Change-Id: I032f62e7440ce4e4daf7495085f7c75877cb5188
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624868
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843549}