Record some metrics in the Memory Pressure dispatcher

Change-Id: I9780fb3a54efd0d8b7c7473574e10f66e5fede10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037452
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739514}

• base/util/memory_pressure/multi_source_memory_pressure_monitor.cc[diff]
• base/util/memory_pressure/multi_source_memory_pressure_monitor.h[diff]
• base/util/memory_pressure/multi_source_memory_pressure_monitor_unittest.cc[diff]
• tools/metrics/histograms/histograms.xml[diff]