Fix uninitialized memory on kIndex8 divisor tests in Gold

Turns out bitmap.eraseColor() does nothing if the bitmap
is kIndex8.  We need a more reliable way to initialize
all of the pixels that we look at in Gold.

The input SkCanvas* is always zero initialized, so let's
only draw pixels to the canvas if we have initialized them.

BUG=skia:

Review URL: https://codereview.chromium.org/1418913002
1 file changed