Implement multi-shape clipPaths using Skia pathops.

Currently, RenderSVGResourceClipper::pathOnlyClipping() bails out on multiple shapes, and we fall
back to mask-based clipping (with all its drawbacks). But thanks to Cary's awesome pathops work,
we can now union arbitrary paths and avoid masking.

All suppressed tests require minor rebaselining.

A note on the transformed-text-fill-pattern.html update: it turns out that this ref-test relies on
mask-based clipping to function properly (text pattern fill alignment issue?) - this can be verified
in the absence of this CL by adjusting the clipPath to only contain one path. To avoid breaking the
ref-test, we can either add an empty <text> element to the clipPath (text forces mask-based
clipping), or drop the clipPath altogether as the comments suggest it was intended to be a
workaround for Ahem alignment issues. I believe the latter is preferable if it turns out that the
workaround is no longer necessary.

This CL depends on https://codereview.chromium.org/18912002/.

R=schenney@chromium.org

Review URL: https://codereview.chromium.org/18178017

git-svn-id: svn://svn.chromium.org/blink/trunk@154124 bbb929c8-8fbe-4397-9dbb-9b2b20218538