Fix drop-shadow filter animations. The problem is that animated filters switch to the accelerated implementation during animations, and drop back to raster afterwards. The existing raster implementation of FEDropShadow was using the ShadowBlur class, which implements the legacy blur fudge factor that WebKit has always implemented (and is wrong). Rather than fix that one (which will affect box-shadow and many more places), this patch just fixes FEDropShadow by using Skia's SkBlurImageFilter instead. This is what we are using on the accelerated path, so the results match. You can see this in the filter-change-repaint test, which uses compositing on one of the samples and raster on the rest (which are too small). With this change, they match.

BUG=266957
R=fmalita@chromium.org

Review URL: https://codereview.chromium.org/21588004

git-svn-id: svn://svn.chromium.org/blink/trunk@155623 bbb929c8-8fbe-4397-9dbb-9b2b20218538