Add flag for SkGpuSurface creation to enable distance fields.

BUG=skia:2173
R=bsalomon@google.com, reed@google.com, bungeman@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/261783004

git-svn-id: http://skia.googlecode.com/svn/trunk@14525 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/gpu/SkGpuDevice.h b/include/gpu/SkGpuDevice.h
index 7f564de..8042ed3 100644
--- a/include/gpu/SkGpuDevice.h
+++ b/include/gpu/SkGpuDevice.h
@@ -32,6 +32,7 @@
     enum Flags {
         kNeedClear_Flag = 1 << 0,  //!< Surface requires an initial clear
         kCached_Flag    = 1 << 1,  //!< Surface is cached and needs to be unlocked when released
+        kDFFonts_Flag   = 1 << 2,  //!< Surface should render fonts using signed distance fields
     };
 
     /**