Attempt to fix -1->unsigned warning on mac 10.6

src/gpu/effects/GrRRectEffect.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrRRectEffect.h"
 
 #include "gl/GrGLEffect.h"
@@ skipping 651 matching lines @@
         // Check for the "tab" cases - two adjacent circular corners and two square corners.
         SkScalar radius = 0;
         cornerFlags = 0;
         for (int c = 0; c < 4; ++c) {
             const SkVector& r = rrect.radii((SkRRect::Corner)c);
             SkASSERT((0 == r.fX) == (0 == r.fY));
             if (0 == r.fX) {
                 continue;
             }
             if (r.fX != r.fY) {
-                cornerFlags = -1;
+                cornerFlags = ~0U;
                 break;
             }
             if (!cornerFlags) {
                 radius = r.fX;
                 if (radius < CircularRRectEffect::kRadiusMin) {
-                    cornerFlags = -1;
+                    cornerFlags = ~0U;
                     break;
                 }
                 cornerFlags = 1 << c;
             } else {
                 if (r.fX != radius) {
-                   cornerFlags = -1;
+                   cornerFlags = ~0U;
                    break;
                 }
                 cornerFlags |= 1 << c;
             }
         }
 
         switch (cornerFlags) {
             case CircularRRectEffect::kTopLeft_CornerFlag:
             case CircularRRectEffect::kTopRight_CornerFlag:
             case CircularRRectEffect::kBottomRight_CornerFlag:
@@ skipping 17 matching lines @@
                                                         rrect);
                     }
                 }
                 return NULL;
         }
     } else {
         return NULL;
     }
     return CircularRRectEffect::Create(edgeType, cornerFlags, rrect);
 }