Add a way for the gpu veto to report back the reason why it said no

BUG=2334
R=bsalomon@google.com

Author: humper@google.com

Review URL: https://codereview.chromium.org/301423002

git-svn-id: http://skia.googlecode.com/svn/trunk/include@15012 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/core/SkPicture.h b/core/SkPicture.h
index 44681ea..7ccd337 100644
--- a/core/SkPicture.h
+++ b/core/SkPicture.h
@@ -296,7 +296,7 @@
      */
 
 #if SK_SUPPORT_GPU
-    bool suitableForGpuRasterization(GrContext*) const;
+    bool suitableForGpuRasterization(GrContext*, const char ** = NULL) const;
 #endif
 
 protected: