[DevTools] Add protocol::Binary to protocol_string.h

This is in preparation of referencing this type from the code
generated by third_party/inspector_protocol/code_generator.py.

My overall WIP change is in here, and happy to explain more:
https://chromium-review.googlesource.com/c/chromium/src/+/1244719

Bug: chromium:891377
Change-Id: I9fbb8b980e522c0063f96a8a2a42498903d445a8
Reviewed-on: https://chromium-review.googlesource.com/c/1282208
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600051}

chrome/browser/devtools/protocol_string.h

@@ -79,6 +79,20 @@ class StringUtil {
   static std::unique_ptr<protocol::Value> parseJSON(const String&);
 };
 
+// A read-only sequence of uninterpreted bytes with reference-counted storage.
+// Though the templates for generating the protocol bindings reference
+// this type, thus far it's not used in the Chrome layer, so we provide no
+// implementation here and rely on the linker optimizing it away. If this
+// changes, look to content/browser/devtools/protocol_string{.h,.cc} for
+// inspiration.
+class Binary {
+ public:
+  const uint8_t* data() const;
+  size_t size() const;
+  String toBase64() const;
+  static Binary createFromBase64(const String& base64, bool* success);
+};
+
 std::unique_ptr<protocol::Value> toProtocolValue(const base::Value* value,
                                                  int depth);
 std::unique_ptr<base::Value> toBaseValue(protocol::Value* value, int depth);