Only update largest image/text when they change.

Change-Id: Ib0995aa8c7c5714edaf400d0f46188cd3be1e536
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129406Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Commit-Queue: Tal Pressman <talp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755192}

third_party/blink/renderer/core/paint/largest_contentful_paint_calculator.cc

@@ -68,11 +68,13 @@ void LargestContentfulPaintCalculator::UpdateLargestContentPaintIfNeeded(
   bool text_has_changed = false;
   if (largest_image.has_value()) {
     image_has_changed = HasLargestImageChanged(largest_image_, *largest_image);
-    OnLargestImageUpdated(*largest_image);
+    if (image_has_changed)
+      OnLargestImageUpdated(*largest_image);
   }
   if (largest_text.has_value()) {
     text_has_changed = HasLargestTextChanged(largest_text_, *largest_text);
-    OnLargestTextUpdated(*largest_text);
+    if (text_has_changed)
+      OnLargestTextUpdated(*largest_text);
   }
   // If |largest_image| does not have value, the detector may have been
   // destroyed. In this case, keep using its last candidate for comparison with