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This also removes some associated plumbing into SiteDetails and moves
tests that depended on the old metric to use
Memory.RenderProcessHost.Count.All.  This required tweaking tests a
bit to account for the spare process, which is included in the new
metric but not in the old one.

Bug: 1059823
Change-Id: I1a03b9c6acfe8e02b6a479aca8cdebf13a71220d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095974Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749713}