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This adds OnGraphCreated test seams to the browser harness, as well as
a utility function for running a lambda synchronously in the graph
(a common operation in tests).

Change-Id: I4c10dbba34bc8c1d81722ac3780491079a8cadf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493687Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Auto-Submit: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821281}