Unprefix -webkit-clip-path

This CL renames '-webkit-clip-path' to 'clip-path', and makes the former
an alias of the latter.
For LayoutSVGRoot, clip-path is now applied only by the PaintLayer and
not by SVGPaintContext.

Intent-to-ship:

https://groups.google.com/a/chromium.org/d/topic/blink-dev/FBE05hzCmPo

BUG=633028
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2312713002
Cr-Commit-Position: refs/heads/master@{#418827}
27 files changed