Lazily prune the multibuffer block cache.

media/blink/multibuffer.h

Index: media/blink/multibuffer.h
diff --git a/media/blink/multibuffer.h b/media/blink/multibuffer.h
index cecca4a2c2f988328607a4a26def8620bec9a91f..61a3a7ed5783f5edadf77edf4755542eba3a506e 100644
--- a/media/blink/multibuffer.h
+++ b/media/blink/multibuffer.h
@@ -137,6 +137,9 @@ class MEDIA_BLINK_EXPORT MultiBuffer {
     // Virtual for testing purposes.
     void Prune(int64_t max_to_free);
 
+    // Returns true if some blocks are prunable.
+    bool Pruneable() const;
+
     void IncrementDataSize(int64_t blocks);
     void IncrementMaxSize(int64_t blocks);
 
@@ -151,12 +154,21 @@ class MEDIA_BLINK_EXPORT MultiBuffer {
     friend class base::RefCounted<GlobalLRU>;
     ~GlobalLRU();
 
+    // Schedule background pruning, if needed.
+    void SchedulePrune();
+
+    // Perform background pruning.
+    void PruneTask();
+
     // Max number of blocks.
     int64_t max_size_;
 
     // Sum of all multibuffer::data_.size().
     int64_t data_size_;
 
+    // True if there is a call to the background pruning outstanding.
+    bool background_pruning_active_;
+
     // The LRU should contain all blocks which are not pinned from
     // all multibuffers.
     LRU<GlobalBlockId> lru_;