Prevent AutoscrollForSelection when selecting text in a fixed-position element.

AutoscrollForSelection is triggered when selecting text and dragging mouse out
of the container. However, we don't want selecting text in a fixed-position
element triggers autoscroll otherwhere. Inside findAutoscrollable, when we find
the object is inside a fixed-position element, we should prevent
AutoscrollForSelection from happening.

BUG=655489

Review-Url: https://codereview.chromium.org/2441683002
Cr-Commit-Position: refs/heads/master@{#427146}

third_party/WebKit/LayoutTests/fast/events/autoscroll-disabled-in-fix.html

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<div style="height: 400px; position: fixed; width: 400px; top: 500px;">
+<br />Fixed.......................
+<br />Fixed.......................
+<br />Fixed.......................
+<br />Fixed.......................
+<br />Fixed.......................
+<br />Fixed.......................
+<br />Fixed.......................
+</div>
+<div style="height: 2000px"></div>
+<script>
+var testFix = async_test('Selection-autoscroll should not be triggered when the selection is in a fixed-position element');
+testFix.step(function() {
+    if (!window.eventSender)
+        return;
+    var dragStartX = 50;
+    var dragStartY = 550;
+    var dragEndX = 60;
+    var dragMoveY = 580;
+    var dragEndY = 820;
+
+    eventSender.dragMode = false;
+    eventSender.mouseMoveTo(dragStartX, dragStartY);
+    eventSender.mouseDown();
+    eventSender.mouseMoveTo(dragEndX, dragMoveY);
+    eventSender.mouseMoveTo(dragEndX, dragEndY);
+
+    requestAnimationFrame(function() {
+        assert_equals(document.body.scrollTop, 0);
+        testFix.done();
+    });
+});
+</script>

third_party/WebKit/Source/core/layout/LayoutBox.cpp

@@ -1008,6 +1008,12 @@ LayoutBox* LayoutBox::findAutoscrollable(LayoutObject* layoutObject) {
   while (
       layoutObject &&
       !(layoutObject->isBox() && toLayoutBox(layoutObject)->canAutoscroll())) {
+    // Do not start autoscroll when the node is inside a fixed-position element.
+    if (layoutObject->isBox() && toLayoutBox(layoutObject)->hasLayer() &&
+        toLayoutBox(layoutObject)->layer()->scrollsWithViewport()) {
+      return nullptr;
+    }
+
     if (!layoutObject->parent() &&
         layoutObject->node() == layoutObject->document() &&
         layoutObject->document().localOwner())
@@ -1016,7 +1022,8 @@ LayoutBox* LayoutBox::findAutoscrollable(LayoutObject* layoutObject) {
       layoutObject = layoutObject->parent();
   }
 
-  return layoutObject && layoutObject->isBox() ? toLayoutBox(layoutObject) : 0;
+  return layoutObject && layoutObject->isBox() ? toLayoutBox(layoutObject)
+                                               : nullptr;
 }
 
 static inline int adjustedScrollDelta(int beginningDelta) {