Fix bad Proxy feature test in ES5/3.

The previous test assumed that if Proxy exists, Proxy.create also
exists, which is not necessarily the case these days.

@ 78af8b82e56f9cab2f899d0ef22c912afb603810

[MarkM, 2016-03-09 18:55:40]

LGTM

https://codereview.appspot.com/292850043/diff/1/src/com/google/caja/es53.js
File src/com/google/caja/es53.js (right):

https://codereview.appspot.com/292850043/diff/1/src/com/google/caja/es53.js#n...
src/com/google/caja/es53.js:4399: var nativeProxies = Proxy && Proxy.create &&
(function () {
Please add comment pointing out that Proxy.create is no longer standard. If
there is already and adequate comment elsewhere, feel free to ignore this
request.

[kpreid_google, 2016-03-09 18:59:22]

https://codereview.appspot.com/292850043/diff/1/src/com/google/caja/es53.js
File src/com/google/caja/es53.js (right):

https://codereview.appspot.com/292850043/diff/1/src/com/google/caja/es53.js#n...
src/com/google/caja/es53.js:4399: var nativeProxies = Proxy && Proxy.create &&
(function () {
On 2016/03/09 18:55:40, MarkM wrote:
> Please add comment pointing out that Proxy.create is no longer standard. If
> there is already and adequate comment elsewhere, feel free to ignore this
> request.

This is not the only use of Proxy.create, es53 is deprecated, and we already
have https://github.com/google/caja/issues/1947 for the whole matter of removing
uses of Proxy.create. Is that not sufficient?

[MarkM, 2016-03-09 19:00:07]

On 2016/03/09 18:59:22, kpreid_google wrote:
> https://codereview.appspot.com/292850043/diff/1/src/com/google/caja/es53.js
> File src/com/google/caja/es53.js (right):
> 
>
https://codereview.appspot.com/292850043/diff/1/src/com/google/caja/es53.js#n...
> src/com/google/caja/es53.js:4399: var nativeProxies = Proxy && Proxy.create &&
> (function () {
> On 2016/03/09 18:55:40, MarkM wrote:
> > Please add comment pointing out that Proxy.create is no longer standard. If
> > there is already and adequate comment elsewhere, feel free to ignore this
> > request.
> 
> This is not the only use of Proxy.create, es53 is deprecated, and we already
> have https://github.com/google/caja/issues/1947 for the whole matter of
removing
> uses of Proxy.create. Is that not sufficient?

It is sufficient, thanks.