Check scroll layer when updating composited scroll offset

ScrollingCoordinator::UpdateCompositedScrollOffset() assumes that the
scroll layer exists. We should not call the function when we haven't
created the scroll layer.

The check was not needed before crrev.com/c/1895081 because we created
the layers earilier (during CompositeUpdate). After the CL we created
the layers during PrePaint, but VisualViewport::DidSetScaleOrLocation()
can be called earlier.

Bug: 1020181
Change-Id: Ie9a341928ed13e02b4edbefdfbc513f1ecb22be0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899598
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713141}

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

@@ -542,8 +542,10 @@ bool VisualViewport::DidSetScaleOrLocation(float scale,
 
     // SVG runs with accelerated compositing disabled so no
     // ScrollingCoordinator.
-    if (auto* coordinator = GetPage().GetScrollingCoordinator())
-      coordinator->UpdateCompositedScrollOffset(this);
+    if (auto* coordinator = GetPage().GetScrollingCoordinator()) {
+      if (scroll_layer_)
+        coordinator->UpdateCompositedScrollOffset(this);
+    }
 
     EnqueueScrollEvent();
 
@@ -595,6 +597,7 @@ void VisualViewport::CreateLayers() {
   LayerForScrollingDidChange(coordinator->GetCompositorAnimationTimeline());
 
   InitializeScrollbars();
+  coordinator->UpdateCompositedScrollOffset(this);
 }
 
 void VisualViewport::InitializeScrollbars() {

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

@@ -2696,5 +2696,27 @@ TEST_F(VisualViewportSimTest, UsedColorSchemeFromRootElement) {
   EXPECT_EQ(WebColorScheme::kDark, visual_viewport.UsedColorScheme());
 }
 
+TEST_P(VisualViewportTest, SetLocationBeforePrePaint) {
+  InitializeWithAndroidSettings();
+  WebView()->MainFrameWidget()->Resize(WebSize(100, 100));
+  RegisterMockedHttpURLLoad("content-width-1000.html");
+  NavigateTo(base_url_ + "content-width-1000.html");
+
+  // Simulate that the visual viewport is just created and FrameLoader is
+  // restoring the previously saved scale and scroll state.
+  VisualViewport& visual_viewport = GetFrame()->GetPage()->GetVisualViewport();
+  visual_viewport.DisposeImpl();
+  ASSERT_FALSE(visual_viewport.LayerForScrolling());
+  visual_viewport.SetScaleAndLocation(1.75, false, FloatPoint(12, 34));
+  EXPECT_EQ(FloatPoint(12, 34), visual_viewport.ScrollPosition());
+
+  UpdateAllLifecyclePhases();
+  EXPECT_EQ(FloatPoint(12, 34), visual_viewport.ScrollPosition());
+  // When we create the scrolling layer, we should update its scroll offset.
+  ASSERT_TRUE(visual_viewport.LayerForScrolling());
+  EXPECT_EQ(gfx::ScrollOffset(12, 34),
+            visual_viewport.LayerForScrolling()->CurrentScrollOffset());
+}
+
 }  // namespace
 }  // namespace blink