Fix python style problems indicated by Nils for dmprof visualizer.

BUG=259206
NOTRY=True

Review URL: https://codereview.chromium.org/24935002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225670 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225670