add flag to disable changes to image scaling to avoid breaking every single
layout test on blink all at once.

BUG=
R=epoger@google.com
TBR=epoger
NOTRY=True
NOTREECHECKS=True

Author: humper@google.com

Review URL: https://codereview.chromium.org/298133008

git-svn-id: http://skia.googlecode.com/svn/trunk@14931 2bbb7eff-a529-9590-31e7-b0007b416f81