Bug 1607138 - gXPCOMThreadsShutdown should be atomic r=xpcom-reviewers,mccr8

This commit makes `gXPCOMThreadsShutdown` atomic. I've deliberated on this one for a while because I was mostly interested in how timer threads may be trying to init during shutdown, but these aren't the only places where we are making accesses into `gXPCOMThreadsShutdown` so it should be made atomic regardless.

Differential Revision: https://phabricator.services.mozilla.com/D102486

xpcom/build/XPCOMInit.cpp

@@ -148,7 +148,8 @@ nsresult nsLocalFileConstructor(nsISupports* aOuter, const nsIID& aIID,
 
 nsComponentManagerImpl* nsComponentManagerImpl::gComponentManager = nullptr;
 bool gXPCOMShuttingDown = false;
-bool gXPCOMThreadsShutDown = false;
+mozilla::Atomic<bool, mozilla::SequentiallyConsistent> gXPCOMThreadsShutDown(
+    false);
 bool gXPCOMMainThreadEventsAreDoomed = false;
 char16_t* gGREBinPath = nullptr;
 

xpcom/build/nsXPCOM.h

@@ -10,6 +10,7 @@
 #include "nscore.h"
 #include "nsXPCOMCID.h"
 #include "mozilla/Attributes.h"
+#include "mozilla/Atomics.h"
 
 #ifdef __cplusplus
 #  define DECL_CLASS(c) class c
@@ -31,7 +32,8 @@ DECL_CLASS(nsIDebug2);
 
 #ifdef __cplusplus
 extern bool gXPCOMShuttingDown;
-extern bool gXPCOMThreadsShutDown;
+extern mozilla::Atomic<bool, mozilla::SequentiallyConsistent>
+    gXPCOMThreadsShutDown;
 extern bool gXPCOMMainThreadEventsAreDoomed;
 #endif