[fsp] Add histograms for reading directories support.

This patch is a part of a bigger CL:
https://codereview.chromium.org/246913003/
in order to avoid reoccuring conflicts while committing via CQ.

NOTRY=True
R=isherman@chromium.org
TBR=isherman@chromium.org
TEST=None
BUG=248427

Review URL: https://codereview.chromium.org/273683004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269127 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed