Add MOJO_{BEGIN,END}_EXTERN_C macros, and use them.

mojo/public/c/system/macros.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_MACROS_H_
 #define MOJO_PUBLIC_C_SYSTEM_MACROS_H_
 
 #include <stddef.h>
 
 // Annotate a variable indicating it's okay if it's unused.
@@ skipping 44 matching lines @@
 #if __cplusplus >= 201103L
 #define MOJO_ALIGNAS(alignment) alignas(alignment)
 #elif defined(__GNUC__)
 #define MOJO_ALIGNAS(alignment) __attribute__((aligned(alignment)))
 #elif defined(_MSC_VER)
 #define MOJO_ALIGNAS(alignment) __declspec(align(alignment))
 #else
 #error "Please define MOJO_ALIGNAS() for your compiler."
 #endif
 
+// Use these to declare functions in C header files so that they'll be usable
+// from C++.
+#if defined(__cplusplus)
+#define MOJO_BEGIN_EXTERN_C extern "C" {
+#define MOJO_END_EXTERN_C }
+#else
+#define MOJO_BEGIN_EXTERN_C
+#define MOJO_END_EXTERN_C
+#endif
+
 #endif  // MOJO_PUBLIC_C_SYSTEM_MACROS_H_