Add C++ example to jni_generator/README.md

This adds a C++ example to the JNI doc as well as some changes to the
instructions for creating native functions.

Bug: 1066173
Change-Id: Ia84f038e009e5b0f7420f3c27a38a176bb34f537
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128226Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Mehran Mahmoudi <mahmoudi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755008}

base/android/jni_generator/README.md

@@ -83,9 +83,13 @@ To add JNI to a class:
    depend on the location of the `generate_jni` BUILD rule that lists your Java
    source code.) Only include this header from a single `.cc` file as the
    header defines functions. That `.cc` must implement your native code by
-   defining functions named `JNI_${OriginalClassName}_${UpperCamelCaseMethod}`
+   defining non-member functions named `JNI_${OriginalClassName}_${UpperCamelCaseMethod}`
+   for static methods and member functions named `${OriginalClassName}::${UpperCamelCaseMethod}`
+   for non-static methods. Member functions need be declared in the header
+   file as well.
 
 Example:
+#### Java
 ```java
 class MyClass {
   // Cannot be private. Must be package or public.
@@ -93,27 +97,45 @@ class MyClass {
   /* package */ interface Natives {
     void foo();
     double bar(int a, int b);
-    // Either the |ClassName| part of the |nativeClassName| parameter name must
+    // Either the |MyClass| part of the |nativeMyClass| parameter name must
     // match the native class name exactly, or the method annotation
-    // @NativeClassQualifiedName("ClassName") must be used.
+    // @NativeClassQualifiedName("MyClass") must be used.
     //
     // If the native class is nested, use
     // @NativeClassQualifiedName("FooClassName::BarClassName") and call the
     // parameter |nativePointer|.
-    void nonStatic(long nativeClassName, NewStyle self);
+    void nonStatic(long nativeMyClass);
   }
 
   void callNatives() {
-    // NewStyleJni is generated by the JNI annotation processor.
-    // Storing NewStyleJni.get() in a field defeats some of the desired R8
+    // MyClassJni is generated by the JNI annotation processor.
+    // Storing MyClassJni.get() in a field defeats some of the desired R8
     // optimizations, but local variables are fine.
-    Natives jni = NewStyleJni.get();
+    Natives jni = MyClassJni.get();
     jni.foo();
     jni.bar(1,2);
-    jni.nonStatic(this, mNativePointer);
+    jni.nonStatic(mNativePointer);
   }
 }
 ```
+#### C++
+```c++
+#include "base/android/jni_android.h"
+#include "<path to BUILD.gn>/<generate_jni target name>/MyClass_jni.h"
+
+class MyClass {
+public:
+  void NonStatic(JNIEnv* env);
+}
+
+// Notice that unlike Java, function names are capitalized in C++.
+// Static function names should follow this format and don't need to be declared.
+void JNI_MyClass_Foo(JNIEnv* env) { ... }
+void JNI_MyClass_Bar(JNIEnv* env, jint a, jint b) { ... }
+
+// Member functions need to be declared.
+void MyClass::NonStatic(JNIEnv* env) { ... }
+```
 
 **Using the 'native' keyword**