Fix error in determining the target position for smooth scroll animation curve

third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html

Index: third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
diff --git a/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html b/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
index 702dbf148ac7116b0dac2896aac2bf5a3e1bed4f..222d0844487550356b5c05e49a43b4a7f3546c00 100644
--- a/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
+++ b/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
@@ -15,6 +15,21 @@
         smooth scrolling on the compositor works as intended (tested via\
         virtual suite virtual/threaded/).");
 
+    function testDiagonalScroll() {
+        // Reset
+        document.scrollingElement.scrollTop = 0;
+        document.scrollingElement.scrollLeft = 0;
+
+        eventSender.mouseMoveTo(20, 20);
+        // Scroll 3 ticks diagonally.
+        eventSender.mouseScrollBy(-3, -3);
+        // Undo 2 ticks in each direction.
+        eventSender.mouseScrollBy(2, 2);
+        // 40px per tick.
+        shouldBecomeEqual("document.scrollingElement.scrollTop == 40 && " +
+          "document.scrollingElement.scrollLeft == 40", "true", finishJSTest);
+    }
+
     function runTest() {
         if (!window.eventSender || !window.internals) {
             finishJSTest();
@@ -27,19 +42,17 @@
         eventSender.mouseMoveTo(20, 20);
         // Scroll down 3 ticks.
         eventSender.mouseScrollBy(0, -1);
-        eventSender.mouseScrollBy(0, -1);
-        eventSender.mouseScrollBy(0, -1);
+        eventSender.mouseScrollBy(0, -2);
         // Scroll right 3 ticks.
         eventSender.mouseScrollBy(-1, 0);
-        eventSender.mouseScrollBy(-1, 0);
-        eventSender.mouseScrollBy(-1, 0);
+        eventSender.mouseScrollBy(-2, 0);
         // Undo 1 tick in each direction.
         eventSender.mouseScrollBy(0, 1);
         eventSender.mouseScrollBy(1, 0);
 
         // 40px per tick.
         shouldBecomeEqual("document.scrollingElement.scrollTop == 80 && " +
-          "document.scrollingElement.scrollLeft == 80", "true", finishJSTest);
+          "document.scrollingElement.scrollLeft == 80", "true", testDiagonalScroll);
     }
 </script>