Bug 1518999 - Implement PerformancePaintTiming for FirstContentfulPaint r=smaug,mstange

Spec: https://w3c.github.io/paint-timing/#sec-PerformancePaintTiming
We only support FirstContentfulPaint at the moment.

Differential Revision: https://phabricator.services.mozilla.com/D66463

layout/generic/nsTextFrame.cpp

@@ -4881,6 +4881,9 @@ void nsTextFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
       TextDecorations textDecs;
       GetTextDecorations(PresContext(), eResolvedColors, textDecs);
       if (!textDecs.HasDecorationLines()) {
+        if (auto* currentPresContext = aBuilder->CurrentPresContext()) {
+          currentPresContext->SetBuiltInvisibleText();
+        }
         return;
       }
     }