Bug 1531535 - Add visual smooth scroll support to nsIPresShell. r=kats

This patch renames nsIPresShell::SetPendingVisualScrollUpdate() to
ScrollToVisual(), and adds an instant vs. smooth option.

SetPendingVisualScrollUpdate() still exists, as a helper for the instant case.

Differential Revision: https://phabricator.services.mozilla.com/D24553

docshell/base/nsDocShell.cpp

@@ -5660,8 +5660,8 @@ nsresult nsDocShell::SetCurScrollPosEx(int32_t aCurHorizontalPos,
   // TODO: If scrollMode == SMOOTH_MSD, this will effectively override that
   // and jump to the target position instantly. A proper solution here would
   // involve giving nsIScrollableFrame a visual viewport smooth scrolling API.
-  shell->SetPendingVisualScrollUpdate(targetPos,
-                                      layers::FrameMetrics::eMainThread);
+  shell->ScrollToVisual(targetPos, layers::FrameMetrics::eMainThread,
+                        nsIPresShell::ScrollMode::eInstant);
 
   return NS_OK;
 }