Fix narrowing conversion errors from r11986

Although this function only gets used for SkMScalar=float, it gets
compiled in when SkMScalar=double.

R=epoger@google.com
BUG=1764

Author: enne@chromium.org

Review URL: https://codereview.chromium.org/49463005

git-svn-id: http://skia.googlecode.com/svn/trunk@11991 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed