Disk cache: Introduce BlockBitmaps for V3.

net/disk_cache/block_files.h

Index: net/disk_cache/block_files.h
===================================================================
--- net/disk_cache/block_files.h	(revision 209202)
+++ net/disk_cache/block_files.h	(working copy)
@@ -49,8 +49,12 @@
 
   // Returns true if the current block file should not be used as-is to store
   // more records. |block_count| is the number of blocks to allocate.
-  bool NeedToGrowBlockFile(int block_count);
+  bool NeedToGrowBlockFile(int block_count) const;
 
+  // Returns true if this block file can be used to store an extra record of
+  // size |block_count|.
+  bool CanAllocate(int block_count) const;
+
   // Returns the number of empty blocks for this file.
   int EmptyBlocks() const;