[doc] logging: explain how to enable it in junit tests

BUG=

Review URL: https://codereview.chromium.org/1581983002

Cr-Commit-Position: refs/heads/master@{#370682}

base/android/java/src/org/chromium/base/Log.java

@@ -17,7 +17,7 @@ import java.util.Locale;
  * the origin of logs, and enable or disable logging in different parts of the code.
  * </p>
  * <p>
- * Usage documentation: {@code //docs/logging.md}.
+ * Usage documentation: {@code //docs/android_logging.md}.
  * </p>
  */
 public class Log {

docs/android_logging.md

@@ -206,3 +206,23 @@ further.
 
 For more, see the [related page on developer.android.com]
 (http://developer.android.com/tools/debugging/debugging-log.html#filteringOutput)
+
+## Logs in JUnit tests
+
+We use [robolectric](http://robolectric.org/) to run our JUnit tests. It
+replaces some of the Android framework classes with "Shadow" classes
+to ensure that we can run our code in a regular JVM. `android.util.Log` is one
+of those replaced classes, and by default calling `Log` methods doesn't print
+anything.
+
+That default is not changed in the normal configuration, but if you need to
+enable logging locally or for a specific test, just add those few lines to your
+test:
+
+```java
+@Before
+public void setUp() {
+  ShadowLog.stream = System.out;
+  //you other setup here
+}
+```