Replaced is_null() with is_valid in SurfaceId and related classes.

cc/surfaces/local_frame_id.h

Index: cc/surfaces/local_frame_id.h
diff --git a/cc/surfaces/local_frame_id.h b/cc/surfaces/local_frame_id.h
index 859e0070e8d339a92403860aa51afd371ac323cb..3d10aaac9c4b703b3d7dff52717fd47d60fbfa4b 100644
--- a/cc/surfaces/local_frame_id.h
+++ b/cc/surfaces/local_frame_id.h
@@ -23,6 +23,8 @@ class LocalFrameId {
   constexpr LocalFrameId(uint32_t local_id, uint64_t nonce)
       : local_id_(local_id), nonce_(nonce) {}
 
+  constexpr bool is_valid() const { return local_id_ != 0; }

[Fady Samuel, 2016/10/20 12:15:36]

I disagree with this. is_valid() means local_id_ != 0 and nonce_ != 0;

[Alex Z., 2016/11/08 21:38:19]

Done.

+
   constexpr bool is_null() const { return local_id_ == 0 && nonce_ == 0; }

[Fady Samuel, 2016/10/20 12:15:36]

Ideally, I'd like to get rid of is_null() because I worry that that could be
misused.

[Alex Z., 2016/11/08 21:38:19]

Done.

 
   constexpr uint32_t local_id() const { return local_id_; }