[SVGDom] Clipped clipPath support

[SVGDom] Clipped clipPath support

ClipPaths can be clipped too, e.g.:

  <clipPath id="clip1" clip-path="url(#clip2)">...</clipPath>

Since we're not really drawing clips but resolving their geometry,
asPath() needs to take composed clipping into account (and intersect as
needed).

R=reed@google.com,robertphillips@google.com,stephana@google.com

Change-Id: I25959e22fe50f72042147cfe6b416b6b9ac20cd4
Reviewed-on: https://skia-review.googlesource.com/5720
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>