Bug 1561488 - null check document in RecvUIResolutionChanged and RecvThemeChanged, r=bdahl

These were the worrisome missing null checks I noticed, and crash-stat shows that there areactually crashes in these two places

Differential Revision: https://phabricator.services.mozilla.com/D64162

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

dom/ipc/BrowserChild.cpp

@@ -3251,11 +3251,15 @@ mozilla::ipc::IPCResult BrowserChild::RecvRequestNotifyAfterRemotePaint() {
 
 mozilla::ipc::IPCResult BrowserChild::RecvUIResolutionChanged(
     const float& aDpi, const int32_t& aRounding, const double& aScale) {
+  nsCOMPtr<Document> document(GetTopLevelDocument());
+  if (!document) {
+    return IPC_OK();
+  }
+
   ScreenIntSize oldScreenSize = GetInnerSize();
   if (aDpi > 0) {
     mPuppetWidget->UpdateBackingScaleCache(aDpi, aRounding, aScale);
   }
-  nsCOMPtr<Document> document(GetTopLevelDocument());
   RefPtr<nsPresContext> presContext = document->GetPresContext();
   if (presContext) {
     presContext->UIResolutionChangedSync();
@@ -3278,8 +3282,12 @@ mozilla::ipc::IPCResult BrowserChild::RecvUIResolutionChanged(
 
 mozilla::ipc::IPCResult BrowserChild::RecvThemeChanged(
     nsTArray<LookAndFeelInt>&& aLookAndFeelIntCache) {
-  LookAndFeel::SetIntCache(aLookAndFeelIntCache);
   nsCOMPtr<Document> document(GetTopLevelDocument());
+  if (!document) {
+    return IPC_OK();
+  }
+
+  LookAndFeel::SetIntCache(aLookAndFeelIntCache);
   RefPtr<nsPresContext> presContext = document->GetPresContext();
   if (presContext) {
     presContext->ThemeChanged();