Fixes bug in ImageView caching of pixels

It needed to lock the pixels before accessing them.

BUG=336156
TEST=see bug
R=pkotwicz@chromium.org

Review URL: https://codereview.chromium.org/184763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254213 0039d316-1c4b-4281-b951-d872f2087c98
[Telemetry] Teach the perf dashboard that less jank is better.

True discrepancy is unit-less. But this is actually a sort of Jank Index (like Speed Index) where we convert a unitless concept into ms.

A completely smooth list of frames would have a jank of 16.67ms.  Outlier frames would raise that.

BUG=
NOTRY=True

Review URL: https://codereview.chromium.org/179123004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254215 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254215