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

ios/chrome/browser/snapshots/snapshot_cache.mm

Index: ios/chrome/browser/snapshots/snapshot_cache.mm
diff --git a/ios/chrome/browser/snapshots/snapshot_cache.mm b/ios/chrome/browser/snapshots/snapshot_cache.mm
index 49bbe14120d4adeef7f2d04d4f7b9089e8036df6..e6223e3759bc9c6feaf85d7ed6ff21898d7d794f 100644
--- a/ios/chrome/browser/snapshots/snapshot_cache.mm
+++ b/ios/chrome/browser/snapshots/snapshot_cache.mm
@@ -7,7 +7,6 @@
 #import <UIKit/UIKit.h>
 
 #include "base/critical_closure.h"
-#include "base/mac/bind_objc_block.h"
 #include "base/files/file_enumerator.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
@@ -206,14 +205,16 @@ 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);

[noyau (Ping after 24h), 2015/05/19 21:36:34]

Why would we retrieve the image if the callback is nil? On phone, it may be used
to prefill the cache, but on iPad this is a very bad idea. At least in this case
don't call the PostTask. 

Ideally can you add a DCHECK(!IsIPadIdiom()||callback) to the method (making
sure nothing calls it that way)?

[sdefresne, 2015/05/20 13:15:12]

Changed the method to be a no-op on iPad if there is no callback. I didn't use
DCHECK(!IsIPadIdiom()) as that may break downstream as the method is part of
public API.

@@ -225,7 +226,9 @@ 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];
   }