Bug 1745996 - Send a memory pressure event when deactivating sessions. r=jnicol,calu

WebRender retains about 50MBs of memory for every window. When switching
between lots of tabs on Android (where 1 tab = 1 window), this can cause
problems as the app will consume a significant amount of memory.

To avoid this problem, we send a memory pressure event whenever a session is
deactivated, which signals to WebRender that it should deallocate the memory.

This message is sent on a delay of 10s to avoid interfering with tab switching,
and we cancel the message if the tab becomes active again before we fire the
memory pressure event.

Co-Authored-By: Cathy Lu <calu@mozilla.com>
Co-Authored-By: Jonathan Almeida [:jonalmeida] <jonalmeida942@gmail.com>

Differential Revision: https://phabricator.services.mozilla.com/D136965

widget/android/nsWindow.cpp

@@ -1117,6 +1117,21 @@ class LayerViewSupport final
     gkWindow->Resize(aLeft, aTop, aWidth, aHeight, /* repaint */ false);
   }
 
+  void NotifyMemoryPressure() {
+    MOZ_ASSERT(NS_IsMainThread());
+    auto acc = mWindow.Access();
+    if (!acc) {
+      return;  // Already shut down.
+    }
+
+    nsWindow* gkWindow = acc->GetNsWindow();
+    if (!gkWindow) {
+      return;
+    }
+
+    gkWindow->mCompositorBridgeChild->SendNotifyMemoryPressure();
+  }
+
   void SetDynamicToolbarMaxHeight(int32_t aHeight) {
     MOZ_ASSERT(NS_IsMainThread());
     auto acc = mWindow.Access();