[iOS] Fixed tab snapshot cache refresh issues on iPad.

BUG=463851

Review URL: https://codereview.chromium.org/1141423002

Cr-Commit-Position: refs/heads/master@{#330731}

ios/chrome/browser/snapshots/snapshot_cache.mm

@@ -140,7 +140,8 @@ void ConvertAndSaveGreyImage(
     DCHECK_CURRENTLY_ON_WEB_THREAD(web::WebThread::UI);
     propertyReleaser_SnapshotCache_.Init(self, [SnapshotCache class]);
 
-    // TODO(andybons): In the case where the cache grows, it is expensive.
+    if (!IsIPadIdiom()) {
+      // TODO(jbbegue): In the case where the cache grows, it is expensive.
       // Make sure this doesn't suck when there are more than ten tabs.
       imageDictionary_.reset(
           [[NSMutableDictionary alloc] initWithCapacity:kCacheInitialCapacity]);
@@ -160,10 +161,12 @@ void ConvertAndSaveGreyImage(
                  name:UIApplicationDidBecomeActiveNotification
                object:nil];
     }
+  }
   return self;
 }
 
 - (void)dealloc {
+  if (!IsIPadIdiom()) {
     [[NSNotificationCenter defaultCenter]
         removeObserver:self
                   name:UIApplicationDidReceiveMemoryWarningNotification
@@ -176,6 +179,7 @@ void ConvertAndSaveGreyImage(
         removeObserver:self
                   name:UIApplicationDidBecomeActiveNotification
                 object:nil];
+  }
   [super dealloc];
 }
 
@@ -195,6 +199,11 @@ void ConvertAndSaveGreyImage(
                          callback:(void (^)(UIImage*))callback {
   DCHECK_CURRENTLY_ON_WEB_THREAD(web::WebThread::UI);
   DCHECK(sessionID);
+  // iPad does not cache images, so if there is no callback we can avoid an
+  // expensive read from storage.
+  if (IsIPadIdiom() && !callback)
+    return;
+
   UIImage* img = [imageDictionary_ objectForKey:sessionID];
   if (img) {
     if (callback)
@@ -205,14 +214,16 @@ void ConvertAndSaveGreyImage(
   base::PostTaskAndReplyWithResult(
       web::WebThread::GetMessageLoopProxyForThread(
           web::WebThread::FILE_USER_BLOCKING).get(),
-      FROM_HERE,
-      base::BindBlock(^base::scoped_nsobject<UIImage>() {
+      FROM_HERE, base::BindBlock(^base::scoped_nsobject<UIImage>() {
         // Retrieve the image on a high priority thread.
         return base::scoped_nsobject<UIImage>([ReadImageFromDisk(
             [SnapshotCache imagePathForSessionID:sessionID]) retain]);
       }),
       base::BindBlock(^(base::scoped_nsobject<UIImage> image) {
-        if (image)
+        // The iPad tab switcher is currently using its own memory cache so the
+        // image is not stored in memory here if running on iPad.
+        // The same logic is used on image writes (code below).
+        if (!IsIPadIdiom() && image)
           [imageDictionary_ setObject:image forKey:sessionID];
         if (callback)
           callback(image);
@@ -224,7 +235,9 @@ void ConvertAndSaveGreyImage(
   if (!img || !sessionID)
     return;
 
-  // Color snapshots are not used on tablets, so don't keep them in memory.
+  // The iPad tab switcher is currently using its own memory cache so the image
+  // is not stored in memory here if running on iPad.
+  // The same logic is used on image reads (code above).
   if (!IsIPadIdiom()) {
     [imageDictionary_ setObject:img forKey:sessionID];
   }
@@ -346,6 +359,7 @@ void ConvertAndSaveGreyImage(
 }
 
 - (void)handleLowMemory {
+  DCHECK(!IsIPadIdiom());
   DCHECK_CURRENTLY_ON_WEB_THREAD(web::WebThread::UI);
   NSMutableDictionary* dictionary =
       [[NSMutableDictionary alloc] initWithCapacity:2];
@@ -358,11 +372,13 @@ void ConvertAndSaveGreyImage(
 }
 
 - (void)handleEnterBackground {
+  DCHECK(!IsIPadIdiom());
   DCHECK_CURRENTLY_ON_WEB_THREAD(web::WebThread::UI);
   [imageDictionary_ removeAllObjects];
 }
 
 - (void)handleBecomeActive {
+  DCHECK(!IsIPadIdiom());
   DCHECK_CURRENTLY_ON_WEB_THREAD(web::WebThread::UI);
   for (NSString* sessionID in pinnedIDs_)
     [self retrieveImageForSessionID:sessionID callback:nil];