time zone: document non-heap mojo receiver

Most mojo Receiver/Remote objects are getting moved to the heap so they
can be managed by Oilpan. Document exceptions.

Change-Id: I55436d89ff1dd72273e42c1870e0b5c543ba8f92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098152Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751565}

third_party/blink/renderer/core/timezone/timezone_controller.cc

@@ -108,6 +108,9 @@ void TimeZoneController::Init() {
   if (!CanInitializeMojo())
     return;
 
+  // monitor must not use HeapMojoRemote. TimeZoneController is not managed by
+  // Oilpan. monitor is only used to bind receiver_ here and never used
+  // again.
   mojo::Remote<device::mojom::blink::TimeZoneMonitor> monitor;
   Platform::Current()->GetBrowserInterfaceBroker()->GetInterface(
       monitor.BindNewPipeAndPassReceiver());

third_party/blink/renderer/core/timezone/timezone_controller.h

@@ -53,6 +53,8 @@ class CORE_EXPORT TimeZoneController final
   // device::mojom::blink::TimeZoneMonitorClient:
   void OnTimeZoneChange(const String& timezone_id) override;
 
+  // receiver_ must not use HeapMojoReceiver. TimeZoneController is not managed
+  // by Oilpan.
   mojo::Receiver<device::mojom::blink::TimeZoneMonitorClient> receiver_{this};
 
   String host_timezone_id_;