Add option to image_diff to calculate histogram diff

Currently, image_diff will return very high values for the percentage
diff in cases like the entire page getting shifted down by 1px.

This patch adds an alternative inexact way of computing the percentage
diff, which instead diffs the histograms of RGBA values to see whether
the same number of each color pixel occurs. Since the histogram ignores
at what position each pixel occurred, this diff is resistant to shifts,
and even rotation (if you just swap parts of the image, such that the
overall color distribution is the same, the histogram diff will be 0%).

An intended use for this is being able to sort a set of pixel diffs in
order of relative importance.

BUG=135823

Review URL: https://codereview.chromium.org/67973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236516 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236516
Adding base class for cr actions

BUG=316397

Review URL: https://codereview.chromium.org/79523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236517 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236517
app_list: Fix most of the warnings reported by cpplint.py

The report was generated by the following command line:

$ find ui/app_list/ -name '*.cc' -or -name '*.h' | xargs cpplint.py

BUG=None
TEST=None, no behavior changes intended.
R=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/79753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236518 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236518