Exclude video/audio requests from ResourceLoadScheduler throttling

We haven't gotten the breakage reports for these types, but from
what we've observed this could also be a source of breakage, and
we want to be conservative not to break things before enabling this
more broadly.

Bug: 837771
Change-Id: I913e29a9bb0f77dd6cf61f8e4523e89e6d194222
Reviewed-on: https://chromium-review.googlesource.com/1136170Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574863}

third_party/blink/renderer/platform/loader/fetch/resource_loader.cc

@@ -62,9 +62,15 @@ namespace blink {
 namespace {
 
 bool IsThrottlableRequestContext(WebURLRequest::RequestContext context) {
+  // Requests that could run long should not be throttled as they
+  // may stay there forever and avoid other requests from making
+  // progress.
+  // See https://crbug.com/837771 for the sample breakages.
   return context != WebURLRequest::kRequestContextEventSource &&
          context != WebURLRequest::kRequestContextFetch &&
-         context != WebURLRequest::kRequestContextXMLHttpRequest;
+         context != WebURLRequest::kRequestContextXMLHttpRequest &&
+         context != WebURLRequest::kRequestContextVideo &&
+         context != WebURLRequest::kRequestContextAudio;
 }
 
 }  // namespace