[BlobAsync] Adding better error reporting and some new tests.

storage/browser/blob/blob_storage_context.h

Index: storage/browser/blob/blob_storage_context.h
diff --git a/storage/browser/blob/blob_storage_context.h b/storage/browser/blob/blob_storage_context.h
index d72a7cba47e0b2062b3ad42d1ffa23ef8f946cd0..9cf9b6e4ff2ba0196b4a15d74b6fd47f074f1bcc 100644
--- a/storage/browser/blob/blob_storage_context.h
+++ b/storage/browser/blob/blob_storage_context.h
@@ -82,6 +82,7 @@ class STORAGE_EXPORT BlobStorageContext
 
  private:
   using BlobRegistryEntry = BlobStorageRegistry::Entry;
+  using BlobConstructedCallback = BlobStorageRegistry::BlobConstructedCallback;
   friend class content::BlobDispatcherHost;
   friend class BlobAsyncBuilderHost;
   friend class BlobAsyncBuilderHostTest;
@@ -121,9 +122,11 @@ class STORAGE_EXPORT BlobStorageContext
   scoped_ptr<BlobDataSnapshot> CreateSnapshot(const std::string& uuid);
   bool IsBroken(const std::string& uuid) const;
   bool IsBeingBuilt(const std::string& uuid) const;
-  // Runs |done| when construction completes, with true if it was successful.
+  // Runs |done| when construction completes, with true if it was successful,
+  // and false if there was an error, which is reported in the second argument
+  // of the callback.
   void RunOnConstructionComplete(const std::string& uuid,
-                                 const base::Callback<void(bool)>& done);
+                                 const BlobConstructedCallback& done);
 
   // Appends the given blob item to the blob builder.  The new blob
   // retains ownership of data_item if applicable, and returns false if there