[omnibox] Fix OmniboxPopupContentsView crash observing stale widgets

We were making too strong of assumptions about widget vs. view
lifetimes, leading to crashes.

This CL loosens those assumptions (correctly) since we only have a weak
pointer to the widget.

Bug: 1108762
Change-Id: I5eddfe7f5e9dc4e4f51c3caf35056fc2dc388989
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316445
Auto-Submit: Tommy Li <tommycli@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791401}

chrome/browser/ui/views/omnibox/omnibox_popup_contents_view.cc

@@ -474,8 +474,11 @@ void OmniboxPopupContentsView::FireAXEventsForNewActiveDescendant(
 void OmniboxPopupContentsView::OnWidgetBoundsChanged(
     views::Widget* widget,
     const gfx::Rect& new_bounds) {
-  DCHECK_EQ(popup_.get(), widget);
-  DCHECK(popup_);
+  // Because we don't directly control the lifetime of the widget, gracefully
+  // handle "stale" notifications by ignoring them. https://crbug.com/1108762
+  if (!popup_ || popup_.get() != widget)
+    return;
+
   // This is called on rotation or device scale change. We have to re-align to
   // the new location bar location.