Remove unused zero_fill() in GrAtlas.cpp

Triggers warnings with some local clang builds.

R=jvanverth@google.com

Review URL: https://codereview.chromium.org/197663003

git-svn-id: http://skia.googlecode.com/svn/trunk@13773 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed