[QRCode Generator] (fuzzer) guard against null-data in inner service.

Remove a line that was an error but did not surface as a bug.
Add some comments around the method in question for readability.

Bug: 1111685
Change-Id: I0bb763a87d9062bd18bccacea9aee130814ee1a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2365394Reviewed-by: Jeffrey Cohen <jeffreycohen@chromium.org>
Commit-Queue: Travis Skare <skare@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801210}

chrome/services/qrcode_generator/qrcode_generator_service_impl.cc

@@ -270,7 +270,8 @@ void QRCodeGeneratorServiceImpl::GenerateQRCode(
       qr.Generate(base::span<const uint8_t>(
           reinterpret_cast<const uint8_t*>(request->data.data()),
           request->data.size()));
-  if (!qr_data) {
+  if (!qr_data || qr_data->data.data() == nullptr ||
+      qr_data->data.size() == 0) {
     // The above check should have caught the too-long-URL case.
     // Remaining errors can be treated as UNKNOWN.
     response->error_code = mojom::QRCodeGeneratorError::UNKNOWN_ERROR;
@@ -287,10 +288,9 @@ void QRCodeGeneratorServiceImpl::GenerateQRCode(
 
   response->data.insert(response->data.begin(), qr_data_span.begin(),
                         qr_data_span.end());
-  response->data_size = {qr_data_span.size(), qr_data_span.size()};
+  response->data_size = {qr_data->qr_size, qr_data->qr_size};
   response->error_code = mojom::QRCodeGeneratorError::NONE;
-  gfx::Size qr_output_data_size = {qr_data->qr_size, qr_data->qr_size};
-  RenderBitmap(qr_data_span.data(), qr_output_data_size, request, &response);
+  RenderBitmap(qr_data_span.data(), response->data_size, request, &response);
 
   std::move(callback).Run(std::move(response));
 }

chrome/services/qrcode_generator/qrcode_generator_service_impl.h

@@ -48,6 +48,14 @@ class QRCodeGeneratorServiceImpl : public mojom::QRCodeGeneratorService {
 
   // Renders the QR code with pixel information in |data| and render parameters
   // in |request|. Result is stored into |response|.
+  // |data| is input data, one element per module, row-major.
+  // |data_size| is the dimensions of |data|, in modules. Currently expected to
+  //     be square, but function should cope with other shapes.
+  // |request| is the mojo service request object to Generate().
+  //     It includes rendering style preferences expressed by the client.
+  // |response| is the mojo service request object to Generate().
+  //     The bitmap will be populated as a response field if requested by the
+  //     client.
   void RenderBitmap(const uint8_t* data,
                     const gfx::Size data_size,
                     const mojom::GenerateQRCodeRequestPtr& request,