[RootLayerScrolling] Delay call to FrameRectsChanged on overflow scroll.

In the non-RLS code path, when the FrameView's scroll offset changes,
pending_scroll_delta_ is modified, and some of the work is delayed until
the next lifecycle update.  Calling FrameRectsChanged() on every overflow
scroll has a significant performance impact, so this patch replicates the
non-RLS behavior of FrameView by postponing the call to FrameRectsChanged.

BUG=770343

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I10cf0782852dcf1c8021a7a62ce0078aae297e39
Reviewed-on: https://chromium-review.googlesource.com/951641
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541288}

third_party/WebKit/Source/core/frame/LocalFrameView.cpp

@@ -207,6 +207,7 @@ LocalFrameView::LocalFrameView(LocalFrame& frame, IntRect frame_rect)
       horizontal_scrollbar_mode_(kScrollbarAuto),
       vertical_scrollbar_mode_(kScrollbarAuto),
       scrollbars_suppressed_(false),
+      root_layer_did_scroll_(false),
       in_update_scrollbars_(false),
       frame_timing_requests_dirty_(true),
       hidden_for_throttling_(false),
@@ -1717,6 +1718,12 @@ bool LocalFrameView::ShouldSetCursor() const {
          page->GetFocusController().IsActive();
 }
 
+void LocalFrameView::NotifyFrameRectsChangedIfNeededRecursive() {
+  ForAllNonThrottledLocalFrameViews([](LocalFrameView& frame_view) {
+    frame_view.NotifyFrameRectsChangedIfNeeded();
+  });
+}
+
 void LocalFrameView::ScrollContentsIfNeededRecursive() {
   ForAllNonThrottledLocalFrameViews(
       [](LocalFrameView& frame_view) { frame_view.ScrollContentsIfNeeded(); });
@@ -3255,7 +3262,11 @@ bool LocalFrameView::UpdateLifecyclePhasesInternal(
       }
 
       if (target_state >= DocumentLifecycle::kCompositingClean) {
-        ScrollContentsIfNeededRecursive();
+        if (RuntimeEnabledFeatures::RootLayerScrollingEnabled()) {
+          NotifyFrameRectsChangedIfNeededRecursive();
+        } else {
+          ScrollContentsIfNeededRecursive();
+        }
 
         frame_->GetPage()->GlobalRootScrollerController().DidUpdateCompositing(
             *this);
@@ -4741,6 +4752,13 @@ void LocalFrameView::ScrollRectToVisibleInRemoteParent(
       params);
 }
 
+void LocalFrameView::NotifyFrameRectsChangedIfNeeded() {
+  if (root_layer_did_scroll_) {
+    root_layer_did_scroll_ = false;
+    FrameRectsChanged();
+  }
+}
+
 void LocalFrameView::ScrollContentsIfNeeded() {
   if (pending_scroll_delta_.IsZero())
     return;

third_party/WebKit/Source/core/frame/LocalFrameView.h

@@ -616,6 +616,9 @@ class CORE_EXPORT LocalFrameView final
   }
   bool ScrollbarsSuppressed() const { return scrollbars_suppressed_; }
 
+  // Indicates the root layer's scroll offset changed since the last frame
+  void SetRootLayerDidScroll() { root_layer_did_scroll_ = true; }
+
   // Methods for converting between this frame and other coordinate spaces.
   // For definitions and an explanation of the varous spaces, please see:
   // http://www.chromium.org/developers/design-documents/blink-coordinate-spaces
@@ -964,6 +967,7 @@ class CORE_EXPORT LocalFrameView final
   ScrollBehavior ScrollBehaviorStyle() const override;
 
   void ScrollContentsIfNeeded();
+  void NotifyFrameRectsChangedIfNeeded();
 
   enum ComputeScrollbarExistenceOption { kFirstPass, kIncremental };
   void ComputeScrollbarExistence(bool& new_has_horizontal_scrollbar,
@@ -1031,6 +1035,7 @@ class CORE_EXPORT LocalFrameView final
       DocumentLifecycle::LifecycleState target_state);
 
   void ScrollContentsIfNeededRecursive();
+  void NotifyFrameRectsChangedIfNeededRecursive();
   void UpdateStyleAndLayoutIfNeededRecursive();
   void PrePaint();
   void PaintTree();
@@ -1247,7 +1252,7 @@ class CORE_EXPORT LocalFrameView final
   IntSize layout_overflow_size_;
 
   bool scrollbars_suppressed_;
-
+  bool root_layer_did_scroll_;
   bool in_update_scrollbars_;
 
   std::unique_ptr<LayoutAnalyzer> analyzer_;

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

@@ -464,9 +464,8 @@ void PaintLayerScrollableArea::UpdateScrollOffset(
     // As a performance optimization, the scroll offset of the root layer is
     // not included in EmbeddedContentView's stored frame rect, so there is no
     // reason to mark the FrameView as needing a geometry update here.
-    // However plugins still need to be notified, so we call FrameRectsChanged.
     if (is_root_layer)
-      frame_view->FrameRectsChanged();
+      frame_view->SetRootLayerDidScroll();
     else
       frame_view->SetNeedsUpdateGeometries();
     UpdateCompositingLayersAfterScroll();