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

components/cronet/android/histogram_manager.h

Index: components/cronet/android/histogram_manager.h
diff --git a/components/cronet/android/histogram_manager.h b/components/cronet/android/histogram_manager.h
deleted file mode 100644
index 94001a482dc138e47c4554e802581b3b793e8570..0000000000000000000000000000000000000000
--- a/components/cronet/android/histogram_manager.h
+++ /dev/null
@@ -1,16 +0,0 @@
-// 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.
-
-#ifndef COMPONENTS_CRONET_ANDROID_CRONET_HISTOGRAM_MANAGER_H_
-#define COMPONENTS_CRONET_ANDROID_CRONET_HISTOGRAM_MANAGER_H_
-
-#include <jni.h>
-
-namespace cronet {
-
-bool HistogramManagerRegisterJni(JNIEnv* env);
-
-}  // namespace cronet
-
-#endif  // COMPONENTS_CRONET_ANDROID_CRONET_HISTOGRAM_MANAGER_H_