Add meminfo UMA collection.

Change-Id: Ief779a5bdc68b8e5bf2f1ed979bf30b50aca8e0f

BUG=chromium-os:13747
TEST=verify that Platform.Meminfo* entries are in about:histograms.

Review URL: http://codereview.chromium.org/6804014
3 files changed
tree: d2d030968b965afe33ad8c4aef97d2961e423799
  1. metrics/