Bug 1907454 - [1/1] Send `ActorDestroyReason` up into telemetry r=ipc-reviewers,nika

File-picker telemetry shows that most failures to open the file dialog
remotely arise in `UtilityProcessHost::OnChannelClosed`. This ultimately
derives from `IProtocol::ActorDisconnected()`, which is called from many
points, mostly generated code.

Rather than jumping in and writing error-propagation code all the way
back up the stack, send what little information we can easily access up
to the file-picker failure telemetry; this should narrow down what
portion of the code we need to look at to determine the root cause of
these failures.

Differential Revision: https://phabricator.services.mozilla.com/D216350

ipc/glue/UtilityProcessHost.cpp

@@ -277,11 +277,15 @@ void UtilityProcessHost::Shutdown() {
   DestroyProcess();
 }
 
-void UtilityProcessHost::OnChannelClosed() {
+void UtilityProcessHost::OnChannelClosed(
+    IProtocol::ActorDestroyReason aReason) {
   MOZ_ASSERT(NS_IsMainThread());
   LOGD("[%p] UtilityProcessHost::OnChannelClosed", this);
 
-  RejectPromise(LaunchError("UtilityProcessHost::OnChannelClosed"));
+  // `aReason` was not originally passed into this function; a value of 0 for
+  // the `why` in telemetry means it's from an older build with no info
+  RejectPromise(
+      LaunchError("UtilityProcessHost::OnChannelClosed", 1 + (long)aReason));
 
   if (!mShutdownRequested && mListener) {
     // This is an unclean shutdown. Notify our listener that we're going away.

ipc/glue/UtilityProcessHost.h

@@ -100,7 +100,7 @@ class UtilityProcessHost final : public mozilla::ipc::GeckoChildProcessHost {
 
   // Called on the main thread when the mUtilityProcessParent actor is shutting
   // down.
-  void OnChannelClosed();
+  void OnChannelClosed(IProtocol::ActorDestroyReason aReason);
 
   // Kill the remote process, triggering IPC shutdown.
   void KillHard(const char* aReason);

ipc/glue/UtilityProcessParent.cpp

@@ -177,7 +177,7 @@ void UtilityProcessParent::ActorDestroy(ActorDestroyReason aWhy) {
     NS_WARNING("Could not get a nsIObserverService, ipc:utility-shutdown skip");
   }
 
-  mHost->OnChannelClosed();
+  mHost->OnChannelClosed(aWhy);
 }
 
 // To ensure that IPDL is finished before UtilityParent gets deleted.