Fix invalidation during painting of scrollbars

Source/platform/scroll/Scrollbar.cpp

Index: Source/platform/scroll/Scrollbar.cpp
diff --git a/Source/platform/scroll/Scrollbar.cpp b/Source/platform/scroll/Scrollbar.cpp
index 78dc927dc3c3f8ed6c2cfff534e96bad9411c0a3..3d627d069583150c3f9ae2617b3e94bb22d87b97 100644
--- a/Source/platform/scroll/Scrollbar.cpp
+++ b/Source/platform/scroll/Scrollbar.cpp
@@ -105,6 +105,15 @@ Scrollbar::~Scrollbar()
 #endif
 }
 
+void Scrollbar::setFrameRect(const IntRect& frameRect)
+{
+    if (frameRect != this->frameRect()) {

[Julien - ping for review, 2015/02/25 01:43:36]

We prefer early return whenever possible.

[Xianzhu, 2015/02/25 01:57:09]

Done.

+        invalidate();

[chrishtr, 2015/02/24 22:38:42]

Is this because widgets don't obey the standard way of doing paint invalidation?

[Xianzhu, 2015/02/24 23:01:56]

Yes. They are not LayoutObjects, so the normal
previousPaintInvalidationRect/needsFullPaintInvalidation mechanisms don't apply
for them.

+        Widget::setFrameRect(frameRect);
+        invalidate();
+    }
+}
+
 ScrollbarOverlayStyle Scrollbar::scrollbarOverlayStyle() const
 {
     return m_scrollableArea ? m_scrollableArea->scrollbarOverlayStyle() : ScrollbarOverlayStyleDefault;