Promote fixed-position elements with compositing descendants.

This code was accidentally removed in an earlier commit (*).
It seems fixed-position elements with compositing descendants
need to be promoted for the existing compositing system to work.

(*) https://chromium-review.googlesource.com/c/chromium/src/+/1407922

Bug: 950418

Change-Id: Iec04435bb1a20a17c54ce4f5ff35f4e00704e6b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574686
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652637}

third_party/blink/renderer/core/paint/compositing/compositing_requirements_updater.cc

@@ -184,6 +184,11 @@ static CompositingReasons SubtreeReasonsForCompositing(
     subtree_reasons |= CompositingReason::kIsolateCompositedDescendants;
   }
 
+  if (layer->GetLayoutObject().StyleRef().GetPosition() == EPosition::kFixed) {
+    subtree_reasons |=
+        CompositingReason::kPositionFixedWithCompositedDescendants;
+  }
+
   // A layer with preserve-3d or perspective only needs to be composited if
   // there are descendant layers that will be affected by the preserve-3d or
   // perspective.

third_party/blink/renderer/platform/graphics/compositing_reasons.cc

@@ -105,10 +105,9 @@ constexpr CompositingReasonStringMap kCompositingReasonsStringMap[] = {
     {CompositingReason::kIsolateCompositedDescendants,
      "isolateCompositedDescendants",
      "Should isolate descendants to apply a blend effect"},
-    {CompositingReason::kPositionFixedOrStickyWithCompositedDescendants,
-     "positionFixedOrStickyWithCompositedDescendants"
-     "Is a position:fixed or position:sticky element with composited "
-     "descendants"},
+    {CompositingReason::kPositionFixedWithCompositedDescendants,
+     "positionFixedWithCompositedDescendants"
+     "Is a position:fixed element with composited descendants"},
     {CompositingReason::kRoot, "root", "Is the root layer"},
     {CompositingReason::kLayerForAncestorClip, "layerForAncestorClip",
      "Secondary layer, applies a clip due to a sibling in the compositing "

third_party/blink/renderer/platform/graphics/compositing_reasons.h

@@ -61,7 +61,7 @@ using CompositingReasons = uint64_t;
   V(Preserve3DWith3DDescendants)                                              \
   V(ReflectionOfCompositedParent)                                             \
   V(IsolateCompositedDescendants)                                             \
-  V(PositionFixedOrStickyWithCompositedDescendants)                           \
+  V(PositionFixedWithCompositedDescendants)                                   \
                                                                               \
   /* The root layer is a special case. It may be forced to be a layer, but it \
   also needs to be a layer if anything else in the subtree is composited. */  \