Don't force layout for scrollTo(0,0).

Source/core/frame/LocalDOMWindow.cpp

Index: Source/core/frame/LocalDOMWindow.cpp
diff --git a/Source/core/frame/LocalDOMWindow.cpp b/Source/core/frame/LocalDOMWindow.cpp
index 0c636c0bd315c613ab58748e3a472d3563458a3d..29797474e489309e0c86ccca7cdfd7eb306add47 100644
--- a/Source/core/frame/LocalDOMWindow.cpp
+++ b/Source/core/frame/LocalDOMWindow.cpp
@@ -1210,7 +1210,8 @@ void LocalDOMWindow::scrollTo(double x, double y) const
     if (!isCurrentlyDisplayedInFrame())
         return;
 
-    document()->updateLayoutIgnorePendingStylesheets();
+    if (x || y)

[Rick Byers, 2015/04/09 17:46:11]

can you add a brief comment saying why?  I.e. 'Force a layout only when it's
possible the specified scroll position isn't valid'

[rune, 2015/04/09 21:45:05]

Done.

+        document()->updateLayoutIgnorePendingStylesheets();
 
     if (std::isnan(x) || std::isnan(y))
         return;
@@ -1224,8 +1225,6 @@ void LocalDOMWindow::scrollTo(const ScrollToOptions& scrollToOptions) const
     if (!isCurrentlyDisplayedInFrame())
         return;
 
-    document()->updateLayoutIgnorePendingStylesheets();
-
     FrameView* view = frame()->view();
     if (!view)
         return;
@@ -1234,6 +1233,13 @@ void LocalDOMWindow::scrollTo(const ScrollToOptions& scrollToOptions) const
     if (!host)
         return;
 
+    if (!scrollToOptions.hasLeft()
+        || !scrollToOptions.hasTop()
+        || scrollToOptions.left()
+        || scrollToOptions.top()) {
+        document()->updateLayoutIgnorePendingStylesheets();
+    }
+
     double scaledX = 0.0;
     double scaledY = 0.0;