[BlobAsync] Faster shortcuttin, make renderer controller leaky & alive.

storage/browser/blob/blob_async_builder_host.h

Index: storage/browser/blob/blob_async_builder_host.h
diff --git a/storage/browser/blob/blob_async_builder_host.h b/storage/browser/blob/blob_async_builder_host.h
index 0b7fff5f7ffbff1358272df7c29b1d6b83d1152f..2407482fc36479f9d1518f65d454333d4b873ba8 100644
--- a/storage/browser/blob/blob_async_builder_host.h
+++ b/storage/browser/blob/blob_async_builder_host.h
@@ -99,8 +99,8 @@ class STORAGE_EXPORT BlobAsyncBuilderHost {
 
   // This removes the BlobBuildingState from our map and flags the blob as
   // broken in the context. This can be called both from our own logic to cancel
-  // the blob, or from the DispatcherHost (Renderer). If the blob isn't being
-  // built then we do nothing.
+  // the blob, or from the DispatcherHost (Renderer). The blob MUST be being
+  // built in this builder.
   // Note: if the blob isn't in the context (renderer dereferenced it before we
   // finished constructing), then we don't bother touching the context.
   void CancelBuildingBlob(const std::string& uuid,
@@ -112,6 +112,8 @@ class STORAGE_EXPORT BlobAsyncBuilderHost {
   // are any references being held by anyone.
   void CancelAll(BlobStorageContext* context);
 
+  bool IsEmpty() const { return async_blob_map_.empty(); }
+
   size_t blob_building_count() const { return async_blob_map_.size(); }
 
   bool IsBeingBuilt(const std::string& key) const {