Handle semicolons at end of keySplines value

Firefox has more lenient behavior than we have:

https://bugzilla.mozilla.org/show_bug.cgi?id=705236

Discussion on the w3c list indicates we want said behavior:

http://lists.w3.org/Archives/Public/www-svg/2014Sep/0016.html

Adopting this behavior for a single trailing semicolon
fixes the testcase and makes the parsing code a bit simpler.
animate-keySplines.html tests this behavior.

When using more than one semicolon at the end of keySplines
ignore parsing of the value and report an error on the console.
keySplines-parsing-error.html tests this behavior.

BUG=410379

Review URL: https://codereview.chromium.org/539383002

git-svn-id: svn://svn.chromium.org/blink/trunk@182272 bbb929c8-8fbe-4397-9dbb-9b2b20218538