Added an experiment for an LRU snapshot cache.

ios/chrome/browser/snapshots/lru_cache.h

+// 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.
+
+#ifndef IOS_CHROME_BROWSER_SNAPSHOTS_LRU_CACHE_H_
+#define IOS_CHROME_BROWSER_SNAPSHOTS_LRU_CACHE_H_
+
+#import <Foundation/Foundation.h>
+
+// The LRUCache delegate is called before an item is evicted from the cache.
+@protocol LRUCacheDelegate
+
+- (void)lruCacheWillEvictObject:(id<NSObject>)object;
+
+@end
+
+// This class implements a cache with a limited size. Once the cache reach its
+// size limit, it will start to evict items in a Least Recently Used order
+// (where the term "used" is determined in terms of query to the cache).
+@interface LRUCache : NSObject
+
+// The delegate of the LRUCache called when objects are evicted from the cache.
+@property(nonatomic, assign) id<LRUCacheDelegate> delegate;
+
+// The maximum amount of items that the cache can hold before starting to
+// evict. The value 0 is used to signify that the cache can hold an unlimited
+// amount of elements (i.e. never evicts).
+@property(nonatomic, readonly) NSUInteger maxCacheSize;
+
+// Use the initWithCacheSize: designated initializer. The is no good general
+// default value for the cache size.
+- (instancetype)init NS_UNAVAILABLE;
+
+// |maxCacheSize| value is used to specify the maximum amount of items that the
+// cache can hold before starting to evict items.
+- (instancetype)initWithCacheSize:(NSUInteger)maxCacheSize
+    NS_DESIGNATED_INITIALIZER;
+
+// Query the cache for an item corresponding to the |key|. Returns nil if there
+// is no item corresponding to that key.
+- (id)objectForKey:(id<NSObject>)key;
+
+// Adds the pair |key|, |obj| to the cache. If the value of the maxCacheSize
+// property is non zero, the cache may evict an elements if the maximum cache
+// size is reached. If the |key| is already present in the cache, the value for
+// that key is replaced by |object|. For any evicted object and if the delegate
+// is
+// non nil, it will receive a call to the lruCacheWillEvictObject: selector.
+- (void)setObject:(id<NSObject>)object forKey:(NSObject*)key;
+
+// Remove the key, value pair corresponding to the given |key|. If the delegate
+// is non nil, it will receive a call to the lruCacheWillEvictObject: selector.
+- (void)removeObjectForKey:(id<NSObject>)key;
+
+// Remove all objects from the cache. For all evicted objects and if the
+// delegate is non nil, it will receive a call to the lruCacheWillEvictObject:
+// selector.
+- (void)removeAllObjects;
+
+// Returns the amount of items that the cache currently hold.
+- (NSUInteger)count;
+
+// Returns true if the cache is empty.
+- (BOOL)isEmpty;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_SNAPSHOTS_LRU_CACHE_H_