Provide correct saveLayer() bounds in FEGaussianBlur, FEMorphology.

In Skia r13323, I changed Skia's image filter clipping behaviour to
expand clip bounds to accommodate filter margins. However, the legacy
Blink DAG already makes such accommodation, which means we're creating
an offscreen with redundant pixels. The fix is to specify the computed
bounds directly in saveLayer(), which overrides (well, actually
intersects) the clip, giving the correct bounds. This does give
different results for large blur sigmas, but I believe the new results
are spec-correct: the intermediate buffers used by filters should
never exceed the size of the filter region.

Covered by marked svg tests.

BUG=
R=schenney@chromium.org

Review URL: https://codereview.chromium.org/214693008

git-svn-id: svn://svn.chromium.org/blink/trunk@170232 bbb929c8-8fbe-4397-9dbb-9b2b20218538