use new readPixels, as old version using Config8888 is deprecated

BUG=

Review URL: https://codereview.chromium.org/203443004

git-svn-id: svn://svn.chromium.org/blink/trunk@169754 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Cache all the results in SVGSMILElement::repeatCount()

The null-attribute and 'indefinite' cases were not cached, which made
this method do some unnecessary work for each animation update.

BUG=354520

Review URL: https://codereview.chromium.org/206013003

git-svn-id: svn://svn.chromium.org/blink/trunk@169755 bbb929c8-8fbe-4397-9dbb-9b2b20218538