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These histograms record the size of the resident memory per process type and in
total. Resident memory is influenced by factors we control (e.g. memory that is
not accessed can be swapped) and factors we don't control (e.g. an unrelated
process using a lot of memory can force memory in our process to be swapped).
Histograms are recorded once per UMA ping.

These histograms are added to answer the following question:
- Do the efforts we make to reduce the amount of memory accessed by frozen
  renderers has an impact on the size of their working set?

Bug: 885293
Change-Id: I7cb3b1b5bd4d927277f5f4c9e96455dc0ff2ef94
Reviewed-on: https://chromium-review.googlesource.com/c/1301847
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604654}