Use correct snapshot for the tab transition animation on iPad

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

Index: ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.mm
diff --git a/ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.mm b/ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.mm
index 12db3675405241372ef676ba85877956b4998773..0a701cc66fcefb370b017c753f248540c0dc4b00 100644
--- a/ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.mm
+++ b/ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.mm
@@ -568,8 +568,11 @@ enum class SnapshotViewOption {
   base::WeakNSObject<UIImageView> weakTabScreenshotImageView(
       tabScreenshotImageView.get());
 
-  if (self.transitionContext.initialTabModel != tabModel ||
-      transitionContextContent.initialSelectedTabIndex != selectedTabIndex) {
+  if ([self initialTabModelAndTabDifferFromTabModel:tabModel
+                                             andTabID:selectedTab.tabId]) {

[marq (ping after 24h), 2017/03/30 06:17:45]

Align colons. 

+    // If we're transitioning to a different tab than the one we animated in

[marq (ping after 24h), 2017/03/30 06:17:45]

Avoid using "we" in comments; it can be ambiguous. Prefer something like "When
the tab switcher is transitioning to a different tab than the one that was
animated in from ... "

+    // from, generate a new snapshot instead of using the transition
+    // context...
     tabScreenshotImageView.get().image =
         [self updateScreenshotForCellIfNeeded:selectedCell tabModel:tabModel];
     [selectedTab retrieveSnapshot:^(UIImage* snapshot) {
@@ -716,6 +719,17 @@ enum class SnapshotViewOption {
                    completion:completionBlock];
 }
 
+- (BOOL)initialTabModelAndTabDifferFromTabModel:(nonnull TabModel*)tabModel
+                                         andTabID:(nonnull NSString*)tabID {

[marq (ping after 24h), 2017/03/30 06:17:45]

Align colons.

Prefer not to use 'and' to join method keywords; 'and' should only be used to
indicate a method is performing some additional action. This can just be
-initialTabModelAndTabDifferFromTabModel:tabID:

+  BOOL differ = NO;

[marq (ping after 24h), 2017/03/30 06:17:44]

'differs' ? But see below.

+  TabModel *initialTabModel = self.transitionContext.initialTabModel;

[marq (ping after 24h), 2017/03/30 06:17:44]

Chromium style is to have the pointer operator adhere to the type, so this
should be "TabModel* ".

+  NSString* initialTabID = [self transitionContextContentForTabModel:
+                     initialTabModel].initialTabID;
+  differ = initialTabModel != tabModel ||
+    [initialTabID isEqualToString:tabID] == NO;
+  return differ;

[marq (ping after 24h), 2017/03/30 06:17:45]

Instead of using the intermediate variable, it's cleaner here to just return:

return initialTabModel != tabModel || ![initialTabID isEqualToString:tabID];

[marq (ping after 24h), 2017/03/30 09:20:06]

And, (as I should have realized), we can apply De Morgan's Law to simplify this
further to:

return !(initialTabModel == tabModel && [initialTabID isEqualToString:tabID]).

At that point, I wonder if the negation should just be hoisted out of this
method, making it into

- initialTabModelAndTabMatchesTabModel:TabID:

and having the calling code check if (![self  initialTabModelAndTabMatches ...

+}
+
 - (void)updateLocalPanelsCells {
   auto mainTabPanel =
       [self panelControllerForTabModel:[_tabSwitcherModel mainTabModel]];