Add GetFieldID functing to jni_help in the content for XWalk.

Introduced by src@225471

content/browser/android/java/jni_helper.cc

@@ -48,6 +48,16 @@ base::LazyInstance<MethodIDMap> g_method_id_map = LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
+jfieldID GetFieldID(JNIEnv* env,
+                    const base::android::JavaRef<jclass>& clazz,
+                    const char* field_name,
+                    const char* jni_signature) {
+  jfieldID field_id = env->GetFieldID(clazz.obj(), field_name, jni_signature);
+  CHECK(!base::android::ClearException(env) && field_id) <<
+      "Failed to find field " << field_name << " " << jni_signature;
+  return field_id;
+}
+
 jmethodID GetMethodIDFromClassName(JNIEnv* env,
                                    const char* class_name,
                                    const char* method,

content/browser/android/java/jni_helper.h

@@ -7,6 +7,7 @@
 
 #include <jni.h>
 
+#include "base/android/scoped_java_ref.h"
 #include "content/common/content_export.h"
 
 namespace content {
@@ -22,6 +23,13 @@ CONTENT_EXPORT jmethodID GetMethodIDFromClassName(JNIEnv* env,
                                                   const char* method,
                                                   const char* jni_signature);
 
+// Gets the field ID for a class field.
+// This method triggers a fatal assertion if the field could not be found.
+CONTENT_EXPORT jfieldID GetFieldID(JNIEnv* env,
+                                   const base::android::JavaRef<jclass>& clazz,
+                                   const char* field_name,
+                                   const char* jni_signature);
+
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_ANDROID_JAVA_JNI_HELPER_H_