Bug 1535874 - Don't clear out nsStyleSheetService::gInstance when random other nsStyleSheetService objects are destroyed. r=boris

Differential Revision: https://phabricator.services.mozilla.com/D23815

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

layout/base/nsDocumentViewer.cpp

@@ -2299,6 +2299,10 @@ UniquePtr<ServoStyleSet> nsDocumentViewer::CreateStyleSet(Document* aDocument) {
   auto cache = nsLayoutStylesheetCache::Singleton();
   nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance();
 
+  MOZ_ASSERT(sheetService,
+             "should never be creating a StyleSet after the style sheet "
+             "service has gone");
+
   auto styleSet = MakeUnique<ServoStyleSet>();
 
   // User sheets

layout/base/nsStyleSheetService.cpp

@@ -36,14 +36,18 @@ nsStyleSheetService::nsStyleSheetService() {
                 "Convention for Style Sheet");
   NS_ASSERTION(!gInstance,
                "Someone is using CreateInstance instead of GetService");
-  gInstance = this;
+  if (!gInstance) {
+    gInstance = this;
+  }
   nsLayoutStatics::AddRef();
 }
 
 nsStyleSheetService::~nsStyleSheetService() {
   UnregisterWeakMemoryReporter(this);
 
-  gInstance = nullptr;
+  if (gInstance == this) {
+    gInstance = nullptr;
+  }
   nsLayoutStatics::Release();
 }