Handle negative scale in SkDropShadowImageFilter::onFilterNodeBounds.

Add unit tests for positive and negative scale factors for the two effects that
apply a Gaussian blur like this: blur and drop-shadow. The last two assertions
fail without the fix in SkDropShadowImageFilter.

BUG=chromium:600821
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2114313002

Review-Url: https://codereview.chromium.org/2114313002
2 files changed