[chromeos] Fix null-dereference in GetGeolocationFromResponse()

SimpleURLLoader does not return the response body by default for most
of error situations. The failing code was unconditionally
dereferencing the std::unique_ptr<std::string<>> holding the response
body and so, causing a crash.

Bots didn't complain when the CL landed because apparently log levels
greater than 0 are filtered out for unit tests (the line crashing was
a VLOG(1)). That's why an unit test is not provided.

Bug: 876627
Change-Id: Ibbc87955343fbab069cbc90b926593ffe6568ecd
Reviewed-on: https://chromium-review.googlesource.com/1196888Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#587961}

chromeos/geolocation/simple_geolocation_request.cc

@@ -494,9 +494,9 @@ void SimpleGeolocationRequest::OnSimpleURLLoaderComplete(
   }
   RecordUmaResponseCode(response_code);
 
-  const bool parse_success =
-      GetGeolocationFromResponse(is_success, response_code, *response_body,
-                                 simple_url_loader_->GetFinalURL(), &position_);
+  const bool parse_success = GetGeolocationFromResponse(
+      is_success, response_code, response_body ? *response_body : std::string(),
+      simple_url_loader_->GetFinalURL(), &position_);
   // Note that SimpleURLLoader doesn't return a body for non-2xx
   // responses by default.
   const bool server_error =