Shut up some Mac (xcode 3) warnings

Review URL: http://codereview.appspot.com/6503053/



git-svn-id: http://skia.googlecode.com/svn/trunk@5402 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/utils/SkDeferredCanvas.cpp b/src/utils/SkDeferredCanvas.cpp
index 81ebcbd..bbd3ae8 100644
--- a/src/utils/SkDeferredCanvas.cpp
+++ b/src/utils/SkDeferredCanvas.cpp
@@ -470,7 +470,7 @@
             this->flushPending();
             // Free as much as possible to avoid oscillating around fMaxRecordingStorageBytes
             // which could cause a high flushing frequency.
-            this->freeMemoryIfPossible(~0);
+            this->freeMemoryIfPossible(~0U);
         }
         storageAllocated = this->storageAllocatedForRecording();
     }