Push HTTP requests to front of the queue rather than the end.

This is because we should retrieve the oldest requests from the end of
the queue.

Bug: 916420
Change-Id: I0232eb73d2c922297833a93adb22da39c91047e2
Reviewed-on: https://chromium-review.googlesource.com/c/1493339Reviewed-by: Michael Martis <martis@chromium.org>
Commit-Queue: Rachel Wong <wrong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636681}

services/image_annotation/annotator.cc

@@ -420,7 +420,7 @@ void Annotator::OnJpgImageDataReceived(
   local_processors_.erase(source_id);
 
   // Schedule an HTTP request for this image.
-  http_request_queue_.push_back({source_id, image_bytes});
+  http_request_queue_.push_front({source_id, image_bytes});
   pending_source_ids_.insert(source_id);
 
   // Start sending batches to the server.

services/image_annotation/annotator_unittest.cc

@@ -64,8 +64,8 @@ constexpr char kBatchRequest[] = R"(
 {
   "imageRequests": [
     {
-      "imageId": "https://www.example.com/image1.jpg",
-      "imageBytes": "AQID",
+      "imageId": "https://www.example.com/image3.jpg",
+      "imageBytes": "BwgJ",
       "engineParameters": [
         {"ocrParameters": {}},
         {"descriptionParameters": {}}
@@ -80,8 +80,8 @@ constexpr char kBatchRequest[] = R"(
       ]
     },
     {
-      "imageId": "https://www.example.com/image3.jpg",
-      "imageBytes": "BwgJ",
+      "imageId": "https://www.example.com/image1.jpg",
+      "imageBytes": "AQID",
       "engineParameters": [
         {"ocrParameters": {}},
         {"descriptionParameters": {}}