Fix crash on AssistantWebView destroy.

AssistantWebView observed |content_view_| but did not stop observing it
prior to its destruction. When |content_view_| was destroyed, it would
attempt to notify AssistantWebView which, in the case of this bug, was
already destroyed itself.

Note that |content_view_| is owned by ManagedWebContents in
chrome/browser.

Bug: b:116246256
Change-Id: I5839f60a6ba4c1ba07f0ce0977cdb6e0c4f48158
Reviewed-on: https://chromium-review.googlesource.com/1237412
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593342}

ash/assistant/ui/assistant_web_view.cc

@@ -182,6 +182,7 @@ void AssistantWebView::ReleaseWebContents() {
     return;
 
   if (content_view_) {
+    content_view_->RemoveObserver(this);
     RemoveChildView(content_view_);
 
     // In Mash, |content_view_| was owned by the view hierarchy prior to its