Remove SVGViewElement.viewTarget attribute

As per SVG2.0, SVGViewElement.viewTarget is removed.
https://github.com/w3c/svgwg/commit/c1690ae1b99aa95c648a304ec7a8f7dec46d7c83

Intent to Deprecate and Remove:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/X3kyDbj9xlA

BUG=633908

Review-Url: https://codereview.chromium.org/2488703003
Cr-Commit-Position: refs/heads/master@{#432144}
15 files changed