Blob: Call callback with an empty vector when the size of side data is zero

disk_cache::Entry::GetDataSize() can return zero. If the size of
side data is zero, it doesn't make sense to try to read side data.
Call ReadSideDataCallback with an empty vector when the size is zero.
I don't expect this improves performance in general though.

Change-Id: I07237a1b41a4b95349a430806122270084f8b55d
Reviewed-on: https://chromium-review.googlesource.com/c/1307029Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605286}

storage/browser/blob/blob_impl.cc

@@ -151,6 +151,10 @@ void BlobImpl::ReadSideData(ReadSideDataCallback callback) {
         }
         int32_t body_size =
             entry->GetDataSize(item->disk_cache_side_stream_index());
+        if (body_size == 0) {
+          std::move(callback).Run(std::vector<uint8_t>());
+          return;
+        }
         auto io_buffer = base::MakeRefCounted<net::IOBufferWithSize>(body_size);
 
         auto io_callback = base::AdaptCallbackForRepeating(base::BindOnce(