Ensure FragmentData::UniqueId is only called when an object will have an id

FragmentData::UniqueId is not intended to be relied upon as an
indicator. Instead, any call to the method should only happen if you
know there should be a UniqueId, and failures should be handled by
figuring out when a UniqueId should have been created (by paint
properties or otherwise). This CL adds a DCHECK to enforce this, and
fixes up two locations that were trying to rely on
FragmentData::UniqueId to check state.

Bug: None
Change-Id: I168ad2e91a3418e98d351df8fa3ad879ab2e3427
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730859Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684361}

third_party/blink/renderer/core/animation/compositor_animations.cc

@@ -197,13 +197,6 @@ CompositorAnimations::CheckCanStartEffectOnCompositor(
 
   LayoutObject* layout_object = target_element.GetLayoutObject();
   if (paint_artifact_compositor) {
-    // If we are going to check that we can animate these below, we need
-    // to have the UniqueID to compute the target ID.  Let's check it
-    // once in common in advance.
-    if (!layout_object || !layout_object->UniqueId()) {
-      reasons |= kTargetHasInvalidCompositingState;
-    }
-
     // Elements with subtrees containing will-change: contents are not
     // composited for animations as if the contents change the tiles
     // would need to be rerastered anyways.
@@ -308,8 +301,14 @@ CompositorAnimations::CheckCanStartEffectOnCompositor(
       }
 
       if (paint_artifact_compositor) {
-        if (!layout_object || !layout_object->UniqueId())
+        // If we don't have paint properties, we won't have a UniqueId to use
+        // for checking here.
+        if (!target_element.GetLayoutObject() ||
+            !target_element.GetLayoutObject()
+                 ->FirstFragment()
+                 .PaintProperties()) {
           continue;
+        }
 
         CompositorElementId target_element_id =
             CompositorElementIdFromUniqueObjectId(

third_party/blink/renderer/core/animation/scroll_timeline_util.cc

@@ -57,8 +57,10 @@ std::unique_ptr<CompositorScrollTimeline> ToCompositorScrollTimeline(
 
 base::Optional<CompositorElementId> GetCompositorScrollElementId(
     const Node* node) {
-  if (!node || !node->GetLayoutObject() || !node->GetLayoutObject()->UniqueId())
+  if (!node || !node->GetLayoutObject() ||
+      !node->GetLayoutObject()->FirstFragment().PaintProperties()) {
     return base::nullopt;
+  }
   return CompositorElementIdFromUniqueObjectId(
       node->GetLayoutObject()->UniqueId(),
       CompositorElementIdNamespace::kScroll);

third_party/blink/renderer/core/animation/scroll_timeline_util_test.cc

@@ -213,7 +213,6 @@ TEST_F(ScrollTimelineUtilTest, GetCompositorScrollElementIdNoUniqueId) {
   SetBodyInnerHTML("<div id='test'></div>");
   Element* test = GetElementById("test");
   ASSERT_TRUE(test->GetLayoutObject());
-  ASSERT_FALSE(test->GetLayoutObject()->UniqueId());
   EXPECT_EQ(GetCompositorScrollElementId(test), base::nullopt);
 }
 

third_party/blink/renderer/core/paint/fragment_data.h

@@ -48,7 +48,8 @@ class CORE_EXPORT FragmentData {
 
   // An id for this object that is unique for the lifetime of the WebView.
   UniqueObjectId UniqueId() const {
-    return rare_data_ ? rare_data_->unique_id : 0;
+    DCHECK(rare_data_);
+    return rare_data_->unique_id;
   }
 
   // The PaintLayer associated with this LayoutBoxModelObject. This can be null