Bug 1555645 - Make PresShell::EventHandler::MaybeFlushThrottledStyles() stop handling it when the PresShell does not have root PreShell nor Document r=smaug

The crash reports just tell us that the crash occur due to referring around
address 0 in `PresShell::EventHandler::MaybeFlushThrottledStyles()`.
Therefore, I'm not sure which is the actual reason of the crashes though,
this patch makes it null-check root `PresShell` and its `Document` before
accessing the latter.

Differential Revision: https://phabricator.services.mozilla.com/D33735

layout/base/PresShell.cpp

@@ -7318,11 +7318,19 @@ nsIFrame* PresShell::EventHandler::MaybeFlushThrottledStyles(
     return aFrameForPresShell;
   }
 
+  PresShell* rootPresShell = mPresShell->GetRootPresShell();
+  if (NS_WARN_IF(!rootPresShell)) {
+    return nullptr;
+  }
+  Document* rootDocument = rootPresShell->GetDocument();
+  if (NS_WARN_IF(!rootDocument)) {
+    return nullptr;
+  }
+
   AutoWeakFrame weakFrameForPresShell(aFrameForPresShell);
   {  // scope for scriptBlocker.
     nsAutoScriptBlocker scriptBlocker;
-    FlushThrottledStyles(mPresShell->GetRootPresShell()->GetDocument(),
-                         nullptr);
+    FlushThrottledStyles(rootDocument, nullptr);
   }
 
   if (weakFrameForPresShell.IsAlive()) {