cc: Clamp the fixed thumb size to be at least zero.

cc/scrollbar_geometry_stub.cc

Index: cc/scrollbar_geometry_stub.cc
diff --git a/cc/scrollbar_geometry_stub.cc b/cc/scrollbar_geometry_stub.cc
index b9f8cf23033d678605f41104289e550aa53b0233..51fc4f934cbf4749d9546263687f45bb8639f665 100644
--- a/cc/scrollbar_geometry_stub.cc
+++ b/cc/scrollbar_geometry_stub.cc
@@ -4,6 +4,8 @@
 
 #include "cc/scrollbar_geometry_stub.h"
 
+#include <cmath>
+
 using WebKit::WebRect;
 using WebKit::WebScrollbar;
 using WebKit::WebScrollbarThemeGeometry;
@@ -31,7 +33,7 @@ int ScrollbarGeometryStub::thumbPosition(WebScrollbar* scrollbar)
 
 int ScrollbarGeometryStub::thumbLength(WebScrollbar* scrollbar)
 {
-    return m_geometry->thumbLength(scrollbar);
+    return std::max(0, m_geometry->thumbLength(scrollbar));
 }
 
 int ScrollbarGeometryStub::trackPosition(WebScrollbar* scrollbar)