Add external data use observer

chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserverAndroid.java

Index: chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserverAndroid.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserverAndroid.java b/chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserverAndroid.java
new file mode 100644
index 0000000000000000000000000000000000000000..379abbf00b3cff0f1cfdfcbd8598284a1dba089b
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserverAndroid.java
@@ -0,0 +1,51 @@
+// Copyright 2015 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.
+
+package org.chromium.chrome.browser.datausage;
+
+import android.content.Context;
+
+import org.chromium.base.annotations.CalledByNative;
+import org.chromium.base.annotations.JNINamespace;
+import org.chromium.chrome.browser.ChromeApplication;
+import org.chromium.chrome.browser.util.NonThreadSafe;
+
+/**
+ * This class provides a base class implementation of a data use observer that is external to
+ * Chromium. This class is not thread safe.

[newt (away), 2015/10/12 18:01:18]

Based on the commit message, this class will only be used on the IO thread. This
should be clearly documented, since it differs from the vast majority of Java
classes in src/chrome, which are used only on the UI thread. (Also, if there's
not a good reason for keeping this on the IO thread, moving this to run on the
UI thread would reduce the likelihood of bugs.)

[tbansal1, 2015/10/12 22:39:13]

It is easier if we keep it on IO thread since the native code will
also be on IO thread. Added comment.

+ */
+@JNINamespace("chrome::android")
+public class ExternalDataUseObserverAndroid {
+    private static final Object LOCK = new Object();
+    private static NonThreadSafe sThreadCheck = null;

[newt (away), 2015/10/12 18:01:18]

Most classes in Chrome aren't thread safe, and we usually don't add thread
checks. If the class has a tricky or non-standard threading situation, then it's
reasonable to add thread checks.

[tbansal1, 2015/10/12 22:39:13]

Removed.

+    private static ExternalDataUseObserverAndroid sExternalDataUseObserverAndroid;

[newt (away), 2015/10/12 18:01:18]

Avoid statics, especially if they contain non-trivial member variables. A static
object lives for the entire lifetime of the JVM, so it can (and will) outlive
the browsing session, the profile, etc. If the static object happens to have a
(direct or indirect) reference to an activity or profile or window etc, that
will cause a serious memory leak.

[tbansal1, 2015/10/12 22:39:14]

Removed.

+
+    @CalledByNative
+    private static ExternalDataUseObserverAndroid create(Context context, long nativePtr) {
+        synchronized (LOCK) {
+            if (sExternalDataUseObserverAndroid == null) {
+                assert sThreadCheck == null;
+                assert sExternalDataUseObserverAndroid == null;
+                sThreadCheck = new NonThreadSafe();
+
+                sExternalDataUseObserverAndroid =
+                        ((ChromeApplication) context)
+                                .createExternalDataUseObserverAndroid(nativePtr);
+            }
+        }
+        return sExternalDataUseObserverAndroid;
+    }
+
+    @CalledByNative
+    private void onDataUse(String tag, long bytesDownloaded, long bytesUploaded) {
+        assert sThreadCheck.calledOnValidThread();
+    }
+
+    /**
+     * Creates an instance of |@link #sExternalDataUseObserverAndroid}.
+     */
+    public ExternalDataUseObserverAndroid() {

[newt (away), 2015/10/12 18:01:18]

looks like this should be protected

[tbansal1, 2015/10/12 22:39:13]

It complains because ChromeApplication is unable to access it.

+        assert sThreadCheck.calledOnValidThread();
+    }
+}