Fire the callback for all code paths for ChangeListLoader::LoadIfNeeded.

Previously the passed callback was only called if we actually decided
to do the load, now we always call it. This is to support chaining
together async loading operations. I have no insight into why the
callback is dropped in the original code.

Bug: 715355
Change-Id: I5c40f862c0894ce5261880482b5da87ccaf6edfa
Reviewed-on: https://chromium-review.googlesource.com/1098737
Commit-Queue: Stuart Langley <slangley@chromium.org>
Reviewed-by: Sasha Morrissey <sashab@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567603}

components/drive/change_list_loader_unittest.cc

@@ -198,6 +198,15 @@ TEST_F(ChangeListLoaderTest, Load) {
   ResourceEntry entry;
   EXPECT_EQ(FILE_ERROR_OK,
             metadata_->GetResourceEntryByPath(file_path, &entry));
+
+  // Calling LoadIfNeeded a second time is a no-op, ensure that the
+  // callback is called.
+  error = FILE_ERROR_FAILED;
+  change_list_loader_->LoadIfNeeded(
+      google_apis::test_util::CreateCopyResultCallback(&error));
+  EXPECT_FALSE(change_list_loader_->IsRefreshing());
+  base::RunLoop().RunUntilIdle();
+  EXPECT_EQ(FILE_ERROR_OK, error);
 }
 
 TEST_F(ChangeListLoaderTest, Load_LocalMetadataAvailable) {

components/drive/chromeos/change_list_loader.cc

@@ -277,6 +277,9 @@ void ChangeListLoader::LoadIfNeeded(const FileOperationCallback& callback) {
   // If the metadata is not yet loaded, start loading.
   if (!loaded_ && !IsRefreshing())
     Load(callback);
+  else
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE, base::Bind(callback, FILE_ERROR_OK));
 }
 
 void ChangeListLoader::Load(const FileOperationCallback& callback) {