Fix the order in which the iOS Tab Switcher cells are initialized.

ios/chrome/browser/ui/tab_switcher/tab_switcher_panel_cell.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/tab_switcher/tab_switcher_panel_cell.h"
 
 #include "base/mac/scoped_nsobject.h"
 #import "ios/chrome/browser/tabs/tab.h"
 #import "ios/chrome/browser/ui/fade_truncated_label.h"
 #import "ios/chrome/browser/ui/image_util.h"
@@ skipping 212 matching lines @@
   [self contentView].accessibilityLabel = tab.title;
   if (tab.favicon) {
     [_favicon setImage:tab.favicon];
   } else {
     [_favicon setImage:NativeImage(IDR_IOS_OMNIBOX_HTTP)];
   }
 
   CGSize snapshotSize = cellSize;
   snapshotSize.height -= tabSwitcherLocalSessionCellTopBarHeight();
   base::WeakNSObject<TabSwitcherLocalSessionCell> weakCell(self);
+  DCHECK([self cache]);

[lpromero, 2017/01/16 18:03:21]

Can you add a comment in the header for setAppearance to require a delegate set?
Would it make sense to DCHECK(self.delegate) in -cache then?

   _currentPendingSnapshotRequest =
       [[self cache] requestSnapshotForTab:tab
                                  withSize:snapshotSize
                           completionBlock:^(UIImage* image) {
                             DCHECK([NSThread isMainThread]);
                             [weakCell setSnapshot:image];
                             _currentPendingSnapshotRequest = {};
                           }];
 }
 
@@ skipping 190 matching lines @@
 - (void)prepareForReuse {
   [_newTabIcon setAlpha:0];
   [_faviconObtainer cancel];
   _faviconObtainer.reset();
   [_raisedButton setAlpha:0];
   [_raisedButton resetState];
   [super prepareForReuse];
 }
 
 @end