WindowBase64::atob() does not remove space characters from input

WindowBase64::atob() does not remove space characters from input as mandated
by the specification (Step 3):
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#dom-windowbase64-atob

As a result, base64 decoding will fail if the input string contains HTML
space characters.

This CL makes our implementation more flexible so that it allows HTML spaces
in the encoded input, which is apparently desired because quite common:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22731

We now have a 100% pass rate on the following test suite:
http://www.w3c-test.org/html/tests/submission/AryehGregor/base64.html

BUG=314682

Review URL: https://codereview.chromium.org/58143002

git-svn-id: svn://svn.chromium.org/blink/trunk@161395 bbb929c8-8fbe-4397-9dbb-9b2b20218538