fix mask error for BW text, where I forgot to look at just the low-8 bits
of the right mask when computing the number of "full" runs.

possibly related to crbug.com/178796
Review URL: https://codereview.chromium.org/13044012

git-svn-id: http://skia.googlecode.com/svn/trunk@8408 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed