Decompose a weird assert.

I got my hands on a list of new warnings coming out of MSVC when run with /analyze.

I scanned over them and didn't see any bugs, but I did notice this is just a weird
way to write this expression. (MSVC /analyze was wary of '=' in a boolean expression.)

BUG=skia:

Review URL: https://codereview.chromium.org/776033002
1 file changed