[Cronet] Make HistogramManager an abstract class to break dependency on native library.

components/cronet/android/histogram_manager.cc

-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "components/cronet/android/histogram_manager.h"
-
-#include <string>
-#include <vector>
-
-#include "base/android/jni_array.h"
-#include "base/metrics/statistics_recorder.h"
-#include "components/metrics/histogram_manager.h"
-
-#include "jni/HistogramManager_jni.h"
-
-namespace cronet {
-
-// Explicitly register static JNI functions.
-bool HistogramManagerRegisterJni(JNIEnv* env) {
-  return RegisterNativesImpl(env);
-}
-
-static void EnsureInitialized(JNIEnv* env, jobject jcaller) {
-  base::StatisticsRecorder::Initialize();
-}
-
-static jbyteArray GetHistogramDeltas(JNIEnv* env, jobject jcaller) {
-  std::vector<uint8> data;
-  if (!metrics::HistogramManager::GetInstance()->GetDeltas(&data))
-    return NULL;
-  return base::android::ToJavaByteArray(env, &data[0], data.size()).Release();
-}
-
-}  // namespace cronet