Desktop aura: Expand what the ViewsDelegate can do to new windows.

Desktop aura: Expand what the ViewsDelegate can do to new windows.

(This is a reland of r173539, adding early bails to some implementations of OnBeforeWidgetInit so that we don't clobber an already set Widget::InitParams::native_widget.)

There are actual cases where we have to show dialogs with no parent or context,
when there are no browser windows. All of the cases that I can find go through
the CreateWindow() interface.

CreateWindow() now explicitly sets InitParams::top_level to true. The
ViewsDelegate now can do arbitrary manipulations on the InitParams so that when
top_level is true and we have no context, we either create a native_widget or
set the context based on desktop type.

BUG=165759
TBR=ben@chromium.org (who did the original review)

First Review URL: https://chromiumcodereview.appspot.com/11578014

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=175131

[commit-bot: I haz the power, 2013-01-03 22:27:24]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/erg@chromium.org/11758002/6002

[commit-bot: I haz the power, 2013-01-04 01:05:07]

Retried try job too often on win_rel for step(s) sync_integration_tests

[commit-bot: I haz the power, 2013-01-04 07:57:38]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/erg@chromium.org/11758002/6002

[commit-bot: I haz the power, 2013-01-04 12:17:21]

Change committed as 175131