Fix callback definition/usage in C++ bindings doc.

Mojo generated code now uses move-only base::OnceCallback
values, instead of const references to base::Callback, so
update the example code to reflect this.

Also add a small note explaining that a dependency needs to
be added from one of the top-level BUILD.gn groups to the new
interface GN target to ensure the compilation instructions
work correctly (otherwise, ninja complains that it doesn't
know about the target).

BUG=None
R=yzshen@chromium.org,sky@chromium.org

Change-Id: I2289af199da7da2bf8ed3ac0bd2f715d4cf8fbf0
Reviewed-on: https://chromium-review.googlesource.com/612387
Commit-Queue: David Turner <digit@chromium.org>
Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512161}

mojo/public/cpp/bindings/README.md

@@ -47,6 +47,12 @@ mojom("interfaces") {
 }
 ```
 
+Ensure that any target that needs this interface depends on it, e.g. with a line like:
+
+```
+   deps += [ '//services/db/public/interfaces' ]
+```
+
 If we then build this target:
 
 ```
@@ -305,9 +311,9 @@ class Logger {
 
   virtual void Log(const std::string& message) = 0;
 
-  using GetTailCallback = base::Callback<void(const std::string& message)>;
+  using GetTailCallback = base::OnceCallback<void(const std::string& message)>;
 
-  virtual void GetTail(const GetTailCallback& callback) = 0;
+  virtual void GetTail(GetTailCallback callback) = 0;
 }
 
 }  // namespace mojom
@@ -335,8 +341,8 @@ class LoggerImpl : public sample::mojom::Logger {
     lines_.push_back(message);
   }
 
-  void GetTail(const GetTailCallback& callback) override {
-    callback.Run(lines_.back());
+  void GetTail(GetTailCallback callback) override {
+    std::move(callback).Run(lines_.back());
   }
 
  private:
@@ -354,7 +360,7 @@ void OnGetTail(const std::string& message) {
   LOG(ERROR) << "Tail was: " << message;
 }
 
-logger->GetTail(base::Bind(&OnGetTail));
+logger->GetTail(base::BindOnce(&OnGetTail));
 ```
 
 Behind the scenes, the implementation-side callback is actually serializing the