Remove SVGCursorElement

This allows significant cleanup of CSSCursorImageValue, so do that too.

Intent-thread:

 https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/r0lnh5Rw23M

TBR=timvolodine@chromium.org
BUG=650598

Review-Url: https://codereview.chromium.org/2522443002
Cr-Commit-Position: refs/heads/master@{#438116}
53 files changed