[ElementTiming] Do not walk objects that cannot be reported

Bug: 942033
Change-Id: Id45597527c69c84296b67b84b26228d1207b59ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648896Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667178}

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

@@ -179,6 +179,17 @@ bool TextPaintTimingDetector::ShouldWalkObject(
   Node* node = object.GetNode();
   if (!node)
     return false;
+  // If we have finished recording Largest Text Paint and the element is a
+  // shadow element or has no elementtiming attribute, then we should not record
+  // its text.
+  if (!is_recording_ltp_) {
+    if (node->IsInShadowTree() || !node->IsElementNode() ||
+        ToElement(node)
+            ->FastGetAttribute(html_names::kElementtimingAttr)
+            .IsEmpty()) {
+      return false;
+    }
+  }
   DOMNodeId node_id = DOMNodeIds::ExistingIdForNode(node);
   if (node_id == kInvalidDOMNodeId)
     return true;