Bug 1618501 - Don't go into the hit-testing tree dumping codepath if we're not actually dumping. r=botond

This avoids doing a full recursive walk of the tree for no purpose unless
the logging is actually enabled.

Differential Revision: https://phabricator.services.mozilla.com/D64657

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

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -642,7 +642,7 @@ APZCTreeManager::UpdateHitTestingTreeImpl(const ScrollNode& aRoot,
   }
 
   APZCTM_LOG("APZCTreeManager (%p)\n", this);
-  if (mRootNode) {
+  if (mRootNode && MOZ_LOG_TEST(sApzMgrLog, LogLevel::Debug)) {
     mRootNode->Dump("  ");
   }
   SendSubtreeTransformsToChromeMainThread(nullptr);