Conditionally create PaintLayer's scrollable area object

third_party/WebKit/LayoutTests/fast/events/mouse-relative-position.html

Index: third_party/WebKit/LayoutTests/fast/events/mouse-relative-position.html
diff --git a/third_party/WebKit/LayoutTests/fast/events/mouse-relative-position.html b/third_party/WebKit/LayoutTests/fast/events/mouse-relative-position.html
index 50d983f773088ac2021cce4c8bd437cce5a9f071..34058c707ff712f0ef85ad0fcd138313a5cb988f 100644
--- a/third_party/WebKit/LayoutTests/fast/events/mouse-relative-position.html
+++ b/third_party/WebKit/LayoutTests/fast/events/mouse-relative-position.html
@@ -98,6 +98,6 @@
 
     shouldBe("simulateElementClick(testElement, [100, 100, 200, 200]);", "'100, 100, 200, 200'");
     shouldBe("simulateElementClick(spanElement, [16, 10, 16, 10]);", "'16, 10, 16, 10'");
-    shouldBe("simulateElementClick(inputElement, [40, 10, 40, 10]);", "'40, 10, 40, 10'");

[Xianzhu, 2016/02/25 19:57:51]

Actually this was [39, 9, 39, 9] but [40, 10, 40, 10] was also accepted because
of TOLERANCE.

With the new code, event.layerX (which exposes implementation details and I
think should not be part of API) changed from 39 to 95 because the input element
no longer create a layer. It unnecessarily created layer because of the
temporary hasOverflowClip==true between LayoutBox::updateFromStyle() and
LayoutTextControlSingleLine::styleDidChange().

+    shouldBe("simulateElementClick(inputElement, [39, 9, 95, 10]);", "'39, 9, 95, 10'");
 </script>
 </html>