Fix radical assumption that 0.5 in double is exactly convertible to float.

BUG=skia:2181

Review URL: https://codereview.chromium.org/195423002

git-svn-id: http://skia.googlecode.com/svn/trunk@13742 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed