Report traffic usage from ResourceLoadScheduler only on the main thread.

* The current histogram code assumes single-threaded usage (hits
  thread kassertion if resource loading from workers frun).
* Currently we classify all non-main-resource requests as subframes, but
  it's not really true if we include non-main-thread traffic.

(This could be a temporary work-around fix)

Bug: 789354
Change-Id: I321cc6a3cbc39cb10c2b4ad8c110760f839a282f
Reviewed-on: https://chromium-review.googlesource.com/805674
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521314}

third_party/WebKit/Source/platform/loader/fetch/ResourceLoadScheduler.cpp

@@ -136,6 +136,9 @@ void ResourceLoadScheduler::TrafficMonitor::OnThrottlingStateChanged(
 
 void ResourceLoadScheduler::TrafficMonitor::Report(
     const ResourceLoadScheduler::TrafficReportHints& hints) {
+  // Currently we only care about stats from frames.
+  if (!IsMainThread())
+    return;
   if (!hints.IsValid())
     return;
 
@@ -220,6 +223,9 @@ void ResourceLoadScheduler::TrafficMonitor::Report(
 }
 
 void ResourceLoadScheduler::TrafficMonitor::ReportAll() {
+  // Currently we only care about stats from frames.
+  if (!IsMainThread())
+    return;
   if (report_all_is_called_)
     return;
   report_all_is_called_ = true;