Call PaintLayer::UpdateLayerPositionsAfterLayout only from layout roots.

Previously, it always called it from the LayoutView, regardless of what
layout changed.

This will be more efficient in typical cases of non-root layouts. It
may be more inefficient in the following cases
a. Two layout roots are present, of which one is an ancestor of the other
(in this case, the new code will update the overlapping layout tree
portions twice).
b. Cases when LocalFrameview::UpdateLayout recurses, due to NeedsLayout
being true after the first one.

If we consider these cases sufficiently rare, then this CL is ok as-is.
Handling (a) and (b) is possible but is harder to implement.

Bug: 462719,970224
Change-Id: Ie65dc73a70bb9362dfa20ff9ad4070e3945d3c56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728717Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683409}

third_party/blink/renderer/core/frame/local_frame_view.cc

@@ -695,6 +695,8 @@ void LocalFrameView::PerformLayout(bool in_subtree_layout) {
           continue;
         LayoutFromRootObject(*root);
 
+        root->PaintingLayer()->UpdateLayerPositionsAfterLayout();
+
         // We need to ensure that we mark up all layoutObjects up to the
         // LayoutView for paint invalidation. This simplifies our code as we
         // just always do a full tree walk.
@@ -764,6 +766,7 @@ void LocalFrameView::UpdateLayout() {
 
   FontCachePurgePreventer font_cache_purge_preventer;
   StyleRetainScope style_retain_scope;
+  bool in_subtree_layout = IsSubtreeLayout();
   {
     base::AutoReset<bool> change_scheduling_enabled(&layout_scheduling_enabled_,
                                                     false);
@@ -776,8 +779,6 @@ void LocalFrameView::UpdateLayout() {
       ClearLayoutSubtreeRootsAndMarkContainingBlocks();
     GetLayoutView()->ClearHitTestCache();
 
-    bool in_subtree_layout = IsSubtreeLayout();
-
     // TODO(crbug.com/460956): The notion of a single root for layout is no
     // longer applicable. Remove or update this code.
     if (in_subtree_layout)
@@ -873,9 +874,8 @@ void LocalFrameView::UpdateLayout() {
 
   frame_timing_requests_dirty_ = true;
 
-  // FIXME: Could find the common ancestor layer of all dirty subtrees and
-  // mark from there. crbug.com/462719
-  GetLayoutView()->EnclosingLayer()->UpdateLayerPositionsAfterLayout();
+  if (!in_subtree_layout)
+    GetLayoutView()->EnclosingLayer()->UpdateLayerPositionsAfterLayout();
 
   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
       TRACE_DISABLED_BY_DEFAULT("blink.debug.layout.trees"), "LayoutTree", this,