Improve jni_generator sample documentation.

base/android/jni_generator/sample_for_tests.cc

Index: base/android/jni_generator/sample_for_tests.cc
diff --git a/base/android/jni_generator/sample_for_tests.cc b/base/android/jni_generator/sample_for_tests.cc
index 6707742ed01a7087be83cc9c38aa5fe77ce9931f..c5ae11241636fab677b0eab7f30387fbc6cdcc3e 100644
--- a/base/android/jni_generator/sample_for_tests.cc
+++ b/base/android/jni_generator/sample_for_tests.cc
@@ -7,8 +7,10 @@
 #include "base/android/jni_android.h"
 #include "base/android/jni_string.h"
 #include "base/android/scoped_java_ref.h"
-#include "jni/SampleForTests_jni.h"
-
+// Generated file for JNI bindings from C++ to Java @CalledByNative methods.
+// Only to be included in .cc file.
+// Name is based on the java file name: *.java -> jni/*_jni.h
+#include "jni/SampleForTests_jni.h"  // Generated by JNI.
 
 using base::android::AttachCurrentThread;
 using base::android::ScopedJavaLocalRef;
@@ -26,6 +28,11 @@ CPPClass::CPPClass() {
 CPPClass::~CPPClass() {
 }
 
+// static
+bool CPPClass::RegisterJNI(JNIEnv* env) {
+  return RegisterNativesImpl(env);  // Generated in SampleForTests_jni.h
+}
+
 void CPPClass::Destroy(JNIEnv* env, jobject obj) {
   delete this;
 }