Bug 911387 - Add calling CompositableBackendSpecificData::ClearData() in Detach(). r=nical

gfx/layers/composite/CompositableHost.h

@@ -269,6 +269,9 @@ public:
       SetCompositor(nullptr);
       mAttached = false;
       mKeepAttached = false;
+      if (mBackendData) {
+        mBackendData->ClearData();
+      }
     }
   }
   bool IsAttached() { return mAttached; }