Ensure BlobReader never returns a null blob_data.

Ensure |blob_data_| is always initialized on successful codepaths even
when OnDataAvailable() is never invoked.

Bug: 863788
Change-Id: I08d0d73fa2a99c3d0b229c05f1c23377eb72f0b3
Reviewed-on: https://chromium-review.googlesource.com/1137956Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575507}

extensions/browser/blob_reader.cc

@@ -72,6 +72,8 @@ void BlobReader::OnDataAvailable(const void* data, size_t num_bytes) {
 
 void BlobReader::OnDataComplete() {
   data_complete_ = true;
+  if (!blob_data_)
+    blob_data_ = std::make_unique<std::string>();
   if (blob_length_)
     Succeeded();
 }