Add contentWindow attribute for <object> elements

Add contentWindow attribute for <object> elements

This allows for e.g. calling postMessage() on embedded SVG files.

BUG=601771

[lfairy, 2016-04-16 10:03:31]

lambda.fairy@gmail.com changed reviewers:
+ rbyers@chromium.org

[lfairy, 2016-04-16 10:05:41]

lambda.fairy@gmail.com changed reviewers:
+ philipj@opera.com
- rbyers@chromium.org

[lfairy, 2016-04-16 10:24:06]

lambda.fairy@gmail.com changed reviewers:
+ haraken@chromium.org, yukishiino@chromium.org

[philipj_slow, 2016-04-18 13:11:24]

Gecko supports this, but oddly enough Edge does not. Edge, like Blink/WebKit
only supports contentDocument and contentWindow on all of frame, iframe and
object, *except* for this one attribute.

It makes good sense to me to expose this, but I do wonder why it's not exposed
in Edge. I've reached out to some Edge folks at CC'd you.

In the meantime, this needs a new test. I'll also start the try bots to show
which tests are failing due to the change, at least some webexposed tests will
need to be updated.

[philipj_slow, 2016-04-21 08:54:07]

On 2016/04/18 13:11:24, philipj_slow wrote:
> Gecko supports this, but oddly enough Edge does not. Edge, like Blink/WebKit
> only supports contentDocument and contentWindow on all of frame, iframe and
> object, *except* for this one attribute.
> 
> It makes good sense to me to expose this, but I do wonder why it's not exposed
> in Edge. I've reached out to some Edge folks at CC'd you.

Justin Rogers at Microsoft confirms that this is just an omission in Edge and
not due to any known compat issue.

> In the meantime, this needs a new test. I'll also start the try bots to show
> which tests are failing due to the change, at least some webexposed tests will
> need to be updated.

If you add a test, this is OK to land.

[lfairy, 2016-05-10 07:49:33]

Sorry about the delay. I've had a lot on my plate lately, and for this reason
I'm closing the CL. I'd be happy if someone else takes up this issue -- afaict
the only thing needed is tests.