Add auto scrollbars if content would be visible without them.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 291ea69ec6a4b0d4fd5d516cae3f9155a72e388a..2ba7106b7c64e89e5baedabaa892ef8a26ce63fc 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1208,16 +1208,18 @@ void PaintLayerScrollableArea::computeScrollbarExistence(
   if (box().hasAutoHorizontalScrollbar()) {
     if (option == ForbidAddingAutoBars)
       needsHorizontalScrollbar &= hasHorizontalScrollbar();
-    needsHorizontalScrollbar &= box().isRooted() &&
-                                this->hasHorizontalOverflow() &&
-                                box().pixelSnappedClientHeight();
+    needsHorizontalScrollbar &=
+        box().isRooted() && this->hasHorizontalOverflow() &&
+        box().pixelSnappedClientHeight() + box().horizontalScrollbarHeight() >
+            0;
   }
 
   if (box().hasAutoVerticalScrollbar()) {
     if (option == ForbidAddingAutoBars)
       needsVerticalScrollbar &= hasVerticalScrollbar();
-    needsVerticalScrollbar &= box().isRooted() && this->hasVerticalOverflow() &&
-                              box().pixelSnappedClientWidth();
+    needsVerticalScrollbar &=
+        box().isRooted() && this->hasVerticalOverflow() &&
+        box().pixelSnappedClientWidth() + box().verticalScrollbarWidth() > 0;
   }
 
   // Look for the scrollbarModes and reset the needs Horizontal & vertical