[Visual Viewport] Fix a new null dereference.

Fixes a null-dereference regression from a recent CL that added
support for scrolling to a position:fixed element when there
a scrollable visual viewport.

Also implements GetSmoothScrollSequencer by getting it from the
main frame if possible.

BUG=992683

Change-Id: I75cbb7e25e1a8289e1405f01d2d9ddfafb785361
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1749549
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Auto-Submit: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686414}

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

@@ -800,6 +800,12 @@ ChromeClient* VisualViewport::GetChromeClient() const {
   return &GetPage().GetChromeClient();
 }
 
+SmoothScrollSequencer* VisualViewport::GetSmoothScrollSequencer() const {
+  if (!MainFrame())
+    return nullptr;
+  return &MainFrame()->GetSmoothScrollSequencer();
+}
+
 void VisualViewport::SetScrollOffset(const ScrollOffset& offset,
                                      ScrollType scroll_type,
                                      ScrollBehavior scroll_behavior,
@@ -832,8 +838,9 @@ PhysicalRect VisualViewport::ScrollIntoView(
     if (params.is_for_scroll_sequence) {
       DCHECK(params.GetScrollType() == kProgrammaticScroll ||
              params.GetScrollType() == kUserScroll);
-      GetSmoothScrollSequencer()->QueueAnimation(this, new_scroll_offset,
-                                                 behavior);
+      if (SmoothScrollSequencer* sequencer = GetSmoothScrollSequencer()) {
+        sequencer->QueueAnimation(this, new_scroll_offset, behavior);
+      }
     } else {
       SetScrollOffset(new_scroll_offset, params.GetScrollType(), behavior,
                       ScrollCallback());

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

@@ -186,6 +186,7 @@ class CORE_EXPORT VisualViewport final
 
   // ScrollableArea implementation
   ChromeClient* GetChromeClient() const override;
+  SmoothScrollSequencer* GetSmoothScrollSequencer() const override;
   void SetScrollOffset(const ScrollOffset&,
                        ScrollType,
                        ScrollBehavior,