[SVG] Implement 'discard' element

Handled 'discard' element according to SVG2 specification:
http://www.w3.org/TR/SVG2/struct.html#DiscardElement

R=pdr@chromium.org, schenney@chromium.org, dschulze@chromium.org, fmalita@chromium.org
BUG=327339

Review URL: https://codereview.chromium.org/99533004

git-svn-id: svn://svn.chromium.org/blink/trunk@164939 bbb929c8-8fbe-4397-9dbb-9b2b20218538