Have pretty_print.py find histograms.xml.

Whenever I need to pretty print histograms.xml, I always forget that
it wants to be run from the directory containing histograms.xml. This
CL fixes that, by having the tool look for the directory from which it's
running.

R=isherman@chromium.org

Review URL: https://codereview.chromium.org/121403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243267 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243267
cc: Combine analysis and raster

This changes the analysis of PicturePiles from optimistic (hoping to
save raster work by checking if a rastered bitmap would be a solid
color) to pessimistic (assuming most bitmaps are not solid colors and
just checking afterwards).  It does this by rastering to both an
AnalysisCanvas and a normal SkCanvas via SkNWayCanvas and then
checking the results afterwards.

R=reveman@chromium.org, senorblanco@chromium.org
BUG=310796

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=234641

Review URL: https://codereview.chromium.org/63443003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243268 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243268