Make atob() / btoa() argument non optional

Make atob() / btoa() argument non optional

According to the latest specification, the argument to atob() / btoa()
should not be optional:
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#windowbase64

This patch makes Blink behave according to the specification. The argument
is also mandatory in Firefox and IE10.

atob() / btoa() are also moved to their own WindowBase64 interface which
the Window interface implements. This does not change the behavior but
this is closer to the specification and it will make exposing those
methods to workers easier later on.

BUG=261126
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=154419

[haraken, 2013-07-17 13:11:55]

Given the spec, Firefox and IE10, LGTM.

[arv (Not doing code reviews), 2013-07-17 16:59:18]

LGTM

It would be nice to split this out of the .cpp/.h files too.
https://code.google.com/p/chromium/issues/detail?id=261226

[do-not-use, 2013-07-17 17:02:55]

On 2013/07/17 16:59:18, arv wrote:
> LGTM
> 
> It would be nice to split this out of the .cpp/.h files too.
> https://code.google.com/p/chromium/issues/detail?id=261226

Sure, I was planning to do that when exposing these methods to workers. There is
already a bug open for that:
http://www.google.com/url?q=https%3A%2F%2Fcode.google.com%2Fp%2Fchromium%2Fis...

[commit-bot: I haz the power, 2013-07-17 17:07:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ch.dumez@sisa.samsung.com/19576002/1

[commit-bot: I haz the power, 2013-07-17 19:12:13]

Change committed as 154419