Bug 1626152 - Clear cached result instead of removing the property. r=dholbert

This saves an allocation.

Differential Revision: https://phabricator.services.mozilla.com/D69030

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

layout/generic/nsFlexContainerFrame.cpp

@@ -1768,7 +1768,9 @@ class nsFlexContainerFrame::CachedFlexItemData {
 
 void nsFlexContainerFrame::MarkCachedFlexMeasurementsDirty(
     nsIFrame* aItemFrame) {
-  aItemFrame->RemoveProperty(CachedFlexItemData::Prop());
+  if (auto* cache = aItemFrame->GetProperty(CachedFlexItemData::Prop())) {
+    cache->mBAxisMeasurement.reset();
+  }
 }
 
 const CachedBAxisMeasurement&