Added an experiment for an LRU snapshot cache.

ios/chrome/browser/snapshots/snapshot_cache.h

 // Copyright 2012 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.
 
 #ifndef IOS_CHROME_BROWSER_SNAPSHOTS_SNAPSHOT_CACHE_H_
 #define IOS_CHROME_BROWSER_SNAPSHOTS_SNAPSHOT_CACHE_H_
 
 #import <UIKit/UIKit.h>
 
 #include "base/mac/objc_property_releaser.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/time/time.h"
+#import "ios/chrome/browser/snapshots/lru_cache.h"
 
 typedef void (^GreyBlock)(UIImage*);
 
 // A singleton providing an in-memory and on-disk cache of tab snapshots.
 // A snapshot is a full-screen image of the contents of the page at the current
 // scroll offset and zoom level, used to stand in for the UIWebView if it has
 // been purged from memory or when quickly switching tabs.
 // Persists to disk on a background thread each time a snapshot changes.
 @interface SnapshotCache : NSObject {
  @private
   // Dictionary to hold color snapshots in memory. n.b. Color snapshots are not
   // kept in memory on tablets.
   base::scoped_nsobject<NSMutableDictionary> imageDictionary_;
+
+  // Cache to hold color snapshots in memory. n.b. Color snapshots are not
+  // kept in memory on tablets. It is used in place of the imageDictionary_ when
+  // the LRU cache snapshot experiment is enabled.
+  base::scoped_nsobject<LRUCache> lruCache_;
+
   // Temporary dictionary to hold grey snapshots for tablet side swipe. This
   // will be nil before -createGreyCache is called and after -removeGreyCache
   // is called.
   base::scoped_nsobject<NSMutableDictionary> greyImageDictionary_;
   NSSet* pinnedIDs_;
 
   // Session ID of most recent pending grey snapshot request.
   base::scoped_nsobject<NSString> mostRecentGreySessionId_;
   // Block used by pending request for a grey snapshot.
   base::scoped_nsprotocol<GreyBlock> mostRecentGreyBlock_;
@@ skipping 47 matching lines @@
 // immediately call back on |callback|. Otherwise, only use |callback| for the
 // most recent caller. The callback is not guaranteed to be called.
 - (void)greyImageForSessionID:(NSString*)sessionID
                      callback:(void (^)(UIImage*))callback;
 
 // Write a grey copy of the snapshot for |sessionID| to disk, but if and only if
 // a color version of the snapshot already exists in memory or on disk.
 - (void)saveGreyInBackgroundForSessionID:(NSString*)sessionID;
 @end
 
+// Additionnal methods that should only be used for tests.
+@interface SnapshotCache (TestingAdditions)
+- (BOOL)hasImageInMemory:(NSString*)sessionID;
+- (BOOL)hasGreyImageInMemory:(NSString*)sessionID;
+- (NSUInteger)lruCacheMaxSize;
+@end
+
 #endif  // IOS_CHROME_BROWSER_SNAPSHOTS_SNAPSHOT_CACHE_H_