cros: Fix crash when using screen casting extension

In crrev.com/547515 I removed this block of code from
ChromeViewsDelegate::CreateNativeWidget:

  if (params->context)
    params->context = params->context->GetRootWindow();

The Screencastify extension uses DesktopMediaPickerDialogView, which
apparently supplies a context window with no parent. We were relying
on the code above to make params->context null so it would trigger a
fallback code path.

It's not clear why that code was added in the first place. It
appeared in https://crrev.com/230793 as part of a revert of some
RLZ refactoring. However, we depend on the code now, and I cannot
guarantee that all browser code supplies a context with a valid
root window, so put it back.

TBR=oshima@chromium.org

Bug: 828626, 311339
Test: none
Change-Id: I70b9a884fdf471d34d626237b72b92325cba5df8
Reviewed-on: https://chromium-review.googlesource.com/994461Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547978}

chrome/browser/ui/views/chrome_views_delegate_chromeos.cc

@@ -75,6 +75,12 @@ ChromeViewsDelegate::GetOpacityForInitParams(
 views::NativeWidget* ChromeViewsDelegate::CreateNativeWidget(
     views::Widget::InitParams* params,
     views::internal::NativeWidgetDelegate* delegate) {
+  // The context should be associated with a root window. If the context has a
+  // null root window (e.g. the context window has no parent) it will trigger
+  // the fallback case below. https://crbug.com/828626 https://crrev.com/230793
+  if (params->context)
+    params->context = params->context->GetRootWindow();
+
   // Classic ash requires a parent or a context that it can use to look up a
   // root window to find a WindowParentingClient. Mash handles window parenting
   // inside ash, see ash::CreateAndParentTopLevelWindow().