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The only user of this interface was the ProcessMemoryMetricsEmitter,
which now simply posts to the PerformanceManager and walks the graph
for the data it needs.

Bug: 910288
Change-Id: Ife9b43a67afeb36084f5e98c7d19740e1c92e7b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1554786
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648669}