Bug 1613531: Eliminate the a11y telemetry thread in favour of background task; r=Jamie

We replace the a11y telemetry thread with a call to the new
`NS_DispatchBackgroundTask` infrastructure. We use `NS_DISPATCH_EVENT_MAY_BLOCK`
because the task performs some I/O to load the instantiator's version info.

I also took the opportunity to eliminate some redundant copying.

Differential Revision: https://phabricator.services.mozilla.com/D61769

--HG--
extra : moz-landing-system : lando

accessible/windows/msaa/Platform.cpp

@@ -319,13 +319,15 @@ static void GatherInstantiatorTelemetry(nsIFile* aClientExe) {
     AppendVersionInfo(aClientExe, value);
   }
 
-  nsCOMPtr<nsIRunnable> runnable(NS_NewRunnableFunction(
-      "a11y::AccumulateInstantiatorTelemetry",
-      [value]() -> void { AccumulateInstantiatorTelemetry(value); }));
+  nsCOMPtr<nsIRunnable> runnable(
+      NS_NewRunnableFunction("a11y::AccumulateInstantiatorTelemetry",
+                             [value = std::move(value)]() -> void {
+                               AccumulateInstantiatorTelemetry(value);
+                             }));
 
   // Now that we've (possibly) obtained version info, send the resulting
   // string back to the main thread to accumulate in telemetry.
-  NS_DispatchToMainThread(runnable);
+  NS_DispatchToMainThread(runnable.forget());
 }
 
 #endif  // defined(MOZ_TELEMETRY_REPORTING) || defined(MOZ_CRASHREPORTER)
@@ -355,13 +357,15 @@ void a11y::SetInstantiator(const uint32_t aPid) {
   gInstantiator = clientExe;
 
 #if defined(MOZ_TELEMETRY_REPORTING) || defined(MOZ_CRASHREPORTER)
-  nsCOMPtr<nsIRunnable> runnable(NS_NewRunnableFunction(
-      "a11y::GatherInstantiatorTelemetry",
-      [clientExe]() -> void { GatherInstantiatorTelemetry(clientExe); }));
+  nsCOMPtr<nsIRunnable> runnable(
+      NS_NewRunnableFunction("a11y::GatherInstantiatorTelemetry",
+                             [clientExe = std::move(clientExe)]() -> void {
+                               GatherInstantiatorTelemetry(clientExe);
+                             }));
 
-  nsCOMPtr<nsIThread> telemetryThread;
-  NS_NewNamedThread("a11y telemetry", getter_AddRefs(telemetryThread),
-                    runnable);
+  DebugOnly<nsresult> rv =
+      NS_DispatchBackgroundTask(runnable.forget(), NS_DISPATCH_EVENT_MAY_BLOCK);
+  MOZ_ASSERT(NS_SUCCEEDED(rv));
 #endif  // defined(MOZ_TELEMETRY_REPORTING) || defined(MOZ_CRASHREPORTER)
 }