Added an experiment for an LRU snapshot cache.

ios/chrome/browser/snapshots/snapshot_cache.h

Index: ios/chrome/browser/snapshots/snapshot_cache.h
diff --git a/ios/chrome/browser/snapshots/snapshot_cache.h b/ios/chrome/browser/snapshots/snapshot_cache.h
index 6932726ee6ea7514f4854c0878f0de40510a6c42..7ee8d7fa2ca97fb9160cdb78a6cbbfec38777691 100644
--- a/ios/chrome/browser/snapshots/snapshot_cache.h
+++ b/ios/chrome/browser/snapshots/snapshot_cache.h
@@ -10,6 +10,7 @@
 #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*);
 
@@ -23,6 +24,12 @@ typedef void (^GreyBlock)(UIImage*);
   // 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.