Mojo C++ bindings: switch the existing usage of StructTraits to use the new data view interface.

mojo/public/cpp/bindings/string_traits_string_piece.h

Index: mojo/public/cpp/bindings/string_traits_string_piece.h
diff --git a/mojo/public/cpp/bindings/string_traits_string_piece.h b/mojo/public/cpp/bindings/string_traits_string_piece.h
index b6018f22fbe8a99d8da775620025aebf84bfb347..af6be893ac006aad361918ad65a0962c6eddd37f 100644
--- a/mojo/public/cpp/bindings/string_traits_string_piece.h
+++ b/mojo/public/cpp/bindings/string_traits_string_piece.h
@@ -12,12 +12,19 @@ namespace mojo {
 
 template <>
 struct StringTraits<base::StringPiece> {
-  // No IsNull() function, which means that base::StringPiece is always
-  // considered as non-null mojom string. We could have let StringPiece
-  // containing a null data pointer map to null mojom string, but
-  // StringPiece::empty() returns true in this case. It seems confusing to mix
-  // the concept of empty and null strings, especially because they mean
-  // different things in mojom.
+  static bool IsNull(const base::StringPiece& input) {

[dcheng, 2016/05/13 04:12:34]

Usually, StringPiece should be passed by value. Any idea if it makes any codegen
difference here? Since this is a simple inlined template, maybe it's not needed?

[yzshen1, 2016/05/13 04:35:08]

It still works if I change this method's signature to
static bool IsNull(base::StringPiece input);
But I think it is good to keep it consistent with other StructTraits, which all
use const& for the IsNull() method. WDYT?
I am not quite sure which do you mean:
(1) the IsNull() method is not needed; or
(2) the whole StringTraits for base::StringPiece is not needed.

WRT (1): Yes, IsNull() could be omitted to mean "never null". I slightly prefer
to define either both IsNull() and SetToNull(), or none of them.

WRT (2): This StringTraits is useful. Without it you cannot return
base::StringPiece from a StructTraits field getter; or read a string from a
DataView input data as base::StringPiece directly. 

Line 39 of
https://codereview.chromium.org/1966933002/diff/80001/mojo/public/cpp/binding...

Line 17 of
https://codereview.chromium.org/1966933002/diff/80001/mojo/public/cpp/binding...

(I will definitely update our doc to talk about things like this. :))

[dcheng, 2016/05/13 06:05:15]

To clarify, my comment was only referring to using StringPiece instead of const
StringPiece& for the parameter name. More documentation is always good though =)

+    // base::StringPiece is always converted to non-null mojom string. We could
+    // have let StringPiece containing a null data pointer map to null mojom
+    // string, but StringPiece::empty() returns true in this case. It seems
+    // confusing to mix the concept of empty and null strings, especially
+    // because they mean different things in mojom.
+    return false;
+  }
+
+  static void SetToNull(base::StringPiece* output) {
+    // Convert null to an "empty" base::StringPiece.
+    output->set(nullptr, 0);
+  }
 
   static size_t GetSize(const base::StringPiece& input) { return input.size(); }
 
@@ -26,10 +33,7 @@ struct StringTraits<base::StringPiece> {
   }
 
   static bool Read(StringDataView input, base::StringPiece* output) {
-    if (!input.is_null())
-      output->set(input.storage(), input.size());
-    else
-      output->set(nullptr, 0);
+    output->set(input.storage(), input.size());
     return true;
   }
 };