Fix crash in StaticHTMLNativeContent when deallocating WKWebView on iOS9

Reset the scrollview delegate before releasing the WKWebView.
Move the |scrollview setDelegate:| to static_html_native_content.mm for symetry.

BUG=678118

Review-Url: https://codereview.chromium.org/2616633003
Cr-Commit-Position: refs/heads/master@{#441967}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -2750,7 +2750,6 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
     OverscrollActionsController* controller =
         [[[OverscrollActionsController alloc]
             initWithScrollView:[nativeContent scrollView]] autorelease];
-    [[nativeContent scrollView] setDelegate:controller];
     [controller setDelegate:self];
     OverscrollStyle style = _isOffTheRecord
                                 ? OverscrollStyle::REGULAR_PAGE_INCOGNITO

ios/chrome/browser/ui/reading_list/offline_page_native_content.mm

@@ -92,6 +92,7 @@
 - (void)restoreOnlineURL {
   web::NavigationItem* item =
       _webState->GetNavigationManager()->GetLastCommittedItem();
+  DCHECK(item && item->GetVirtualURL() == [self virtualURL]);
   item->SetURL([self virtualURL]);
   item->SetVirtualURL([self virtualURL]);
 }

ios/chrome/browser/ui/static_content/static_html_native_content.mm

@@ -68,6 +68,11 @@
                            URL:URL];
 }
 
+- (void)dealloc {
+  [[self scrollView] setDelegate:nil];
+  [super dealloc];
+}
+
 - (void)loadURL:(const GURL&)URL
              referrer:(const web::Referrer&)referrer
            transition:(ui::PageTransition)transition
@@ -85,6 +90,7 @@
 - (void)setOverscrollActionsController:
     (OverscrollActionsController*)controller {
   _overscrollActionsController.reset([controller retain]);
+  [[self scrollView] setDelegate:controller];
 }
 
 #pragma mark -