[SVG] Avoid unnecessarily populating hashmap for repeat(n) event

This is a follow up patch for repeat(n) animation event support.
Instead of maintaining each repeat(n) in the atomic string hashmap,
we just maintain 'repeatn' in the hashmap and store the repeatCount data with the event.

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

Review URL: https://chromiumcodereview.appspot.com/23944012

git-svn-id: svn://svn.chromium.org/blink/trunk@157988 bbb929c8-8fbe-4397-9dbb-9b2b20218538