Add UMA Histogram Manager to Cronet.

components/cronet/android/histogram_manager.cc

Index: components/cronet/android/histogram_manager.cc
diff --git a/components/cronet/android/histogram_manager.cc b/components/cronet/android/histogram_manager.cc
new file mode 100644
index 0000000000000000000000000000000000000000..173dc666ed06085a08f2ce3456d940811cdbb926
--- /dev/null
+++ b/components/cronet/android/histogram_manager.cc
@@ -0,0 +1,31 @@
+// 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 "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 jbyteArray GetHistogramDeltas(JNIEnv* env, jobject jcaller) {
+  std::vector<uint8> data =
+      metrics::HistogramManager::GetInstance()->GetDeltas();

[mef, 2014/12/11 23:11:40]

qq: how big are deltas? Would this result in extra copy of the data?

[ramant (doing other things), 2014/12/12 02:43:05]

Do we do return value optimization on Android? 

Assumed, in C++11, we don't support RVO and changed GetDeltas to accept
std::vector* as argument.

http://en.cppreference.com/w/cpp/language/copy_elision

http://en.wikipedia.org/wiki/Return_value_optimization.

+  if (!data.empty()) {
+    return base::android::ToJavaByteArray(env, &data[0], data.size()).Release();

[mef, 2014/12/11 23:11:40]

nit: no need for curly braces.

[ramant (doing other things), 2014/12/12 02:43:05]

Done.

+  }
+  return NULL;
+}
+
+}  // namespace cronet