[Chromecast] Update memory pressure heuristics
[Chromecast] Update memory pressure heuristics

Now take buffers + cached memory into account when computing pressure
level.  Also generate moderate pressure events.

Note: actual threshold levels are placeholder; we should tune these
based on data, but don't have the correct data collection in place
yet.  This is still a useful change in the short term to avoid
background apps getting killed constantly.

BUG=internal b/26029747
TEST=watched ~1hr of video across 4 popular apps, monitored memory
     levels.  Observed a few moderate pressure events, 0 critical.

Review URL: https://codereview.chromium.org/1912193003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#389616}

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