change backing store cache to be memory-based rather than count

chrome/browser/renderer_host/backing_store_manager.h

Index: chrome/browser/renderer_host/backing_store_manager.h
diff --git a/chrome/browser/renderer_host/backing_store_manager.h b/chrome/browser/renderer_host/backing_store_manager.h
index 3078a9358e92bfa268ab460a9db54f1b917d6c49..625e87fc14e4c5aa6de3f6d93d462f67863d29ed 100644
--- a/chrome/browser/renderer_host/backing_store_manager.h
+++ b/chrome/browser/renderer_host/backing_store_manager.h
@@ -63,6 +63,9 @@ class BackingStoreManager {
   // removed, false if it wasn't found.
   static bool ExpireBackingStoreForTest(RenderWidgetHost* host);
 
+  // Current size in bytes of the backing store cache.
+  static size_t MemorySize();
+
  private:
   // Not intended for instantiation.
   BackingStoreManager() {}