Let viewport transform nodes use CompositingReason::kViewport

Use the new compositing reason instead of kWillChangeTransform to
distinguish from will-change:transform specified in web pages which
implies that the transform may change to any arbitrary value.

Bug: 1114504
Change-Id: I8a1b4e5fa909dd50f3ae6465cb40d9336c9cd640
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367446Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800293}

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

@@ -178,7 +178,7 @@ PaintPropertyChangeType VisualViewport::UpdatePaintPropertyNodesIfNeeded(
     if (scale_ != 1.f)
       state.transform_and_origin = {TransformationMatrix().Scale(scale_)};
     state.flags.in_subtree_of_page_scale = false;
-    state.direct_compositing_reasons = CompositingReason::kWillChangeTransform;
+    state.direct_compositing_reasons = CompositingReason::kViewport;
     state.compositor_element_id = page_scale_element_id_;
 
     if (!page_scale_node_) {
@@ -259,7 +259,7 @@ PaintPropertyChangeType VisualViewport::UpdatePaintPropertyNodesIfNeeded(
     TransformPaintPropertyNode::State state{
         FloatSize(-scroll_position.Width(), -scroll_position.Height())};
     state.scroll = scroll_node_;
-    state.direct_compositing_reasons = CompositingReason::kWillChangeTransform;
+    state.direct_compositing_reasons = CompositingReason::kViewport;
     if (!scroll_translation_node_) {
       scroll_translation_node_ = TransformPaintPropertyNode::Create(
           *page_scale_node_, std::move(state));

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

@@ -46,6 +46,7 @@ using CompositingReasons = uint64_t;
   V(BackdropFilter)                                                           \
   V(RootScroller)                                                             \
   V(XrOverlay)                                                                \
+  V(Viewport)                                                                 \
                                                                               \
   /* Overlap reasons that require knowing what's behind you in paint-order    \
      before knowing the answer. */                                            \