Fix memory leaks in PageOverlay

R=jamesr@chromium.org
BUG=289039

Review URL: https://chromiumcodereview.appspot.com/24075006

git-svn-id: svn://svn.chromium.org/blink/trunk@157629 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove SVGTRefElement

It's used on 0.001% of pages, not implemented by any other major browser
and causes an undue share of implementation/security complexity.

See https://groups.google.com/a/chromium.org/forum/#!searchin/blink-dev/intent$20to$20remove$20svg$20tref%7Csort:relevance/blink-dev/vnpLfBOsZf4/kPtftDTQCdAJ
for the discussion.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/23795004

git-svn-id: svn://svn.chromium.org/blink/trunk@157630 bbb929c8-8fbe-4397-9dbb-9b2b20218538