Make atob() throw an InvalidCharacterError on excess padding characters

According to the latest specification, window.atob() should throw an
InvalidCharacterError on excess padding characters:
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#dom-windowbase64-atob

This behavior is consistent with both Firefox and IE10.

We are now down from 26 failures to 12 on the following test suite:
http://www.w3c-test.org/html/tests/submission/AryehGregor/base64.html

BUG=261200

Review URL: https://chromiumcodereview.appspot.com/19623002

git-svn-id: svn://svn.chromium.org/blink/trunk@154538 bbb929c8-8fbe-4397-9dbb-9b2b20218538