Recalc custom scrollbar style irrespective of root-layer-scrolls flag.

When chrome running on on root-layer-scrolls, page is scaled the style 
is recalculated and thickness is changed. Change in thickness sets 
informs owning renderer to set for childneedsLayout. This layout shall
happen after style reclac is completed, if this sequence is not executed,
then post layout we see the needsForLayout = true, which asserts and
crashes the chrome.

TESTED=fast/scrollbars/custom-scrollbar-thickness-change-on-zoom-crash.html
now passes with root layer scrolling

BUG=522389(https://code.google.com/p/chromium/issues/detail?id=522389#c1)

Review URL: https://codereview.chromium.org/1310603003

git-svn-id: svn://svn.chromium.org/blink/trunk@201356 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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

@@ -235,6 +235,14 @@ void LayoutBox::styleDidChange(StyleDifference diff, const ComputedStyle* oldSty
     if (isDocumentElement() || isBody()) {
         document().view()->recalculateScrollbarOverlayStyle();
         document().view()->recalculateCustomScrollbarStyle();
+        if (LayoutView* layoutView = view()) {
+            if (DeprecatedPaintLayerScrollableArea* scrollableArea = layoutView->scrollableArea()) {
+                if (scrollableArea->horizontalScrollbar() && scrollableArea->horizontalScrollbar()->isCustomScrollbar())
+                    scrollableArea->horizontalScrollbar()->styleChanged();
+                if (scrollableArea->verticalScrollbar() && scrollableArea->verticalScrollbar()->isCustomScrollbar())
+                    scrollableArea->verticalScrollbar()->styleChanged();
+            }
+        }
     }
     updateShapeOutsideInfoAfterStyleChange(*style(), oldStyle);
     updateGridPositionAfterStyleChange(oldStyle);