Add page fault statistics.

BUG=chromium-os:20624
TEST=observe that Platform.PageFaultXXX appear in about:histograms.

Change-Id: Ifc281d31e05102dc4133d133f732b737e19891f1
Reviewed-on: http://gerrit.chromium.org/gerrit/10143
Commit-Ready: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
4 files changed