Bug 1733465 part 4: Convert nsViewManager::gViewManagers to StaticAutoPtr so that we don't have to manually delete it. r=tnikkel

This removes an explicit 'delete', by letting the StaticAutoPtr type handle the
deletion for us automatically and atomically when we clear its value.

Benefits/justification:
* This reduces the potential for double-free bugs.
* This matches existing patterns elsewhere in our code
  (search for e.g. "StaticAutoPtr<nsTArray")
* This reduces (by 1) the number of explicit `delete` calls that we need to
  consider, when auditing the codebase for potential memory safety issues.

Differential Revision: https://phabricator.services.mozilla.com/D127185

view/nsViewManager.cpp

@@ -50,7 +50,7 @@ using namespace mozilla::layers;
 #undef DEBUG_MOUSE_LOCATION
 
 // Weakly held references to all of the view managers
-nsTArray<nsViewManager*>* nsViewManager::gViewManagers = nullptr;
+StaticAutoPtr<nsTArray<nsViewManager*>> nsViewManager::gViewManagers;
 uint32_t nsViewManager::gLastUserEventTime = 0;
 
 nsViewManager::nsViewManager()
@@ -91,7 +91,6 @@ nsViewManager::~nsViewManager() {
   if (gViewManagers->IsEmpty()) {
     // There aren't any more view managers so
     // release the global array of view managers
-    delete gViewManagers;
     gViewManagers = nullptr;
   }
 

view/nsViewManager.h

@@ -424,7 +424,7 @@ class nsViewManager final {
   // from here to public should be static and locked... MMP
 
   // list of view managers
-  static nsTArray<nsViewManager*>* gViewManagers;
+  static mozilla::StaticAutoPtr<nsTArray<nsViewManager*>> gViewManagers;
 };
 
 /**