<webview>: Add test coverage for deferred newwindow attachment.

<webview>: Add test coverage for deferred newwindow attachment.

We can trigger this case by adding a webview that is not in DOM through
newwindow API.

Fix so that deferred attachment would not throw error, this is done by
returning true in attachWindow() method in this case.

BUG=None
Test=Load a <webview> in a chrome app. Make a newwindow request from
that webview. When the newwindow event fires, call e.window.attach
with a <webview> that is not on DOM, this should work as intended,
when the later <webview> is attached to DOM, it should load properly.

Committed: https://crrev.com/ed1bba31ff004bace7b9f479d1e3d75969142fa2
Cr-Commit-Position: refs/heads/master@{#291594}

[Fady Samuel, 2014-08-22 22:34:04]

lgtm.

Might be nice to have a test where we delete the opener while we have an
unattached new window making sure things don't blow up, and the process goes
away.

[lazyboy, 2014-08-22 22:53:38]

On 2014/08/22 22:34:04, Fady Samuel wrote:
> lgtm.
> 
> Might be nice to have a test where we delete the opener while we have an
> unattached new window making sure things don't blow up, and the process goes
> away.

Filed crbug.com/406616.

[lazyboy, 2014-08-23 00:04:15]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2014-08-23 00:05:15]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/lazyboy@chromium.org/497833003/1

[commit-bot: I haz the power, 2014-08-23 06:53:06]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_chromeos_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  linux_chromium_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[commit-bot: I haz the power, 2014-08-23 21:53:01]

Committed patchset #1 (1) as 1d2a45bba01e0ef30211cc96d876ef34d674493b

[commit-bot: I haz the power, 2014-09-10 02:31:32]

Patchset 1 (id:??) landed as
https://crrev.com/ed1bba31ff004bace7b9f479d1e3d75969142fa2
Cr-Commit-Position: refs/heads/master@{#291594}