Added an experiment for an LRU snapshot cache.

ios/chrome/browser/snapshots/lru_cache.mm

Index: ios/chrome/browser/snapshots/lru_cache.mm
diff --git a/ios/chrome/browser/snapshots/lru_cache.mm b/ios/chrome/browser/snapshots/lru_cache.mm
new file mode 100644
index 0000000000000000000000000000000000000000..01092693f2945d5490af2c64e850f439daec92ee
--- /dev/null
+++ b/ios/chrome/browser/snapshots/lru_cache.mm
@@ -0,0 +1,135 @@
+// 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.
+
+#import "ios/chrome/browser/snapshots/lru_cache.h"
+
+#include "base/containers/hash_tables.h"
+#include "base/containers/mru_cache.h"
+#include "base/mac/scoped_nsobject.h"
+#include "base/memory/scoped_ptr.h"
+
+namespace {
+
+class MRUCacheNSObjectDelegate {
+ public:
+  MRUCacheNSObjectDelegate(id<LRUCacheDelegate> delegate)
+      : delegate_(delegate) {}
+
+  MRUCacheNSObjectDelegate(const MRUCacheNSObjectDelegate& other) = default;
+
+  void operator()(const base::scoped_nsprotocol<id<NSObject>>& payload) const {
+    [delegate_ lruCacheWillEvictObject:payload.get()];
+  }
+
+ private:
+  id<LRUCacheDelegate> delegate_;  // Weak.
+};
+
+struct NSObjectEqualTo {
+  bool operator()(const base::scoped_nsprotocol<id<NSObject>>& obj1,
+                  const base::scoped_nsprotocol<id<NSObject>>& obj2) const {
+    return [obj1 isEqual:obj2];
+  }
+};
+
+struct NSObjectHash {
+  std::size_t operator()(
+      const base::scoped_nsprotocol<id<NSObject>>& obj) const {
+    return [obj hash];
+  }
+};
+
+template <class KeyType, class ValueType>
+struct MRUCacheNSObjectHashMap {
+  typedef base::hash_map<KeyType, ValueType, NSObjectHash, NSObjectEqualTo>
+      Type;
+};
+
+class NSObjectMRUCache
+    : public base::MRUCacheBase<base::scoped_nsprotocol<id<NSObject>>,
+                                base::scoped_nsprotocol<id<NSObject>>,
+                                MRUCacheNSObjectDelegate,
+                                MRUCacheNSObjectHashMap> {
+ private:
+  typedef base::MRUCacheBase<base::scoped_nsprotocol<id<NSObject>>,
+                             base::scoped_nsprotocol<id<NSObject>>,
+                             MRUCacheNSObjectDelegate,
+                             MRUCacheNSObjectHashMap> ParentType;
+
+ public:
+  NSObjectMRUCache(typename ParentType::size_type max_size,
+                   const MRUCacheNSObjectDelegate& deletor)
+      : ParentType(max_size, deletor) {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(NSObjectMRUCache);
+};
+
+}  // namespace
+
+@interface LRUCache ()<LRUCacheDelegate>
+@end
+
+@implementation LRUCache {
+  scoped_ptr<NSObjectMRUCache> _cache;
+}
+
+@synthesize delegate = _delegate;
+@synthesize maxCacheSize = _maxCacheSize;
+
+- (instancetype)init {
+  NOTREACHED();  // Use initWithCacheSize: instead.
+  return nil;
+}
+
+- (instancetype)initWithCacheSize:(NSUInteger)maxCacheSize {
+  self = [super init];
+  if (self) {
+    _maxCacheSize = maxCacheSize;
+    MRUCacheNSObjectDelegate cacheDelegateDeletor(self);
+    _cache.reset(new NSObjectMRUCache(self.maxCacheSize, cacheDelegateDeletor));
+  }
+  return self;
+}
+
+- (id)objectForKey:(id<NSObject>)key {
+  base::scoped_nsprotocol<id<NSObject>> keyObj([key retain]);
+  auto it = _cache->Get(keyObj);
+  if (it == _cache->end())
+    return nil;
+  return it->second.get();
+}
+
+- (void)setObject:(id<NSObject>)value forKey:(NSObject*)key {
+  base::scoped_nsprotocol<id<NSObject>> keyObj([key copy]);
+  base::scoped_nsprotocol<id<NSObject>> valueObj([value retain]);
+  _cache->Put(keyObj, valueObj);
+}
+
+- (void)removeObjectForKey:(id<NSObject>)key {
+  base::scoped_nsprotocol<id<NSObject>> keyObj([key retain]);
+  auto it = _cache->Peek(keyObj);
+  if (it != _cache->end())
+    _cache->Erase(it);
+}
+
+- (void)removeAllObjects {
+  _cache->Clear();
+}
+
+- (NSUInteger)count {
+  return _cache->size();
+}
+
+- (BOOL)isEmpty {
+  return _cache->empty();
+}
+
+#pragma mark - Private
+
+- (void)lruCacheWillEvictObject:(id<NSObject>)obj {
+  [self.delegate lruCacheWillEvictObject:obj];
+}
+
+@end