Bug 1525818 - Remove NS_ASSERTION in ImageBridgeParent::GetInstance() r=mattwoodrow

Sometimes NS_ASSERTION was hit during window closing. It happens because of aync architecture. It is better to change the NS_ASSERTION to NS_WARNING.

Differential Revision: https://phabricator.services.mozilla.com/D18920

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

gfx/layers/ipc/ImageBridgeParent.cpp

@@ -360,7 +360,7 @@ already_AddRefed<ImageBridgeParent> ImageBridgeParent::GetInstance(
   MonitorAutoLock lock(*sImageBridgesLock);
   ImageBridgeMap::const_iterator i = sImageBridges.find(aId);
   if (i == sImageBridges.end()) {
-    NS_ASSERTION(false, "Cannot find image bridge for process!");
+    NS_WARNING("Cannot find image bridge for process!");
     return nullptr;
   }
   RefPtr<ImageBridgeParent> bridge = i->second;