Let ChromiumBridge::paintTextField check if the Color object passed in is valid or not.

webkit/api/src/ChromiumBridge.cpp

Index: webkit/api/src/ChromiumBridge.cpp
===================================================================
--- webkit/api/src/ChromiumBridge.cpp	(revision 16584)
+++ webkit/api/src/ChromiumBridge.cpp	(working copy)
@@ -311,9 +311,12 @@
     const IntRect& rect, const Color& color, bool fillContentArea,
     bool drawEdges)
 {
+    // Fallback to white when |color| is invalid.
+    RGBA32 backgroundColor = color.isValid() ? color.rgb() : Color::white;
+
     webKitClient()->themeEngine()->paintTextField(
         gc->platformContext()->canvas(), part, state, classicState, rect,
-        color.rgb(), fillContentArea, drawEdges);
+        backgroundColor, fillContentArea, drawEdges);
 }
 
 void ChromiumBridge::paintTrackbar(