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

components/cronet/android/test/src/org/chromium/cronet_test_apk/CronetTestActivity.java

Index: components/cronet/android/test/src/org/chromium/cronet_test_apk/CronetTestActivity.java
diff --git a/components/cronet/android/test/src/org/chromium/cronet_test_apk/CronetTestActivity.java b/components/cronet/android/test/src/org/chromium/cronet_test_apk/CronetTestActivity.java
index 863d7639169d5bf8dd0bdeaa15dd4f0fde0e472c..34ce1450b5b51f9081b936af8c3ae7d289ea68c2 100644
--- a/components/cronet/android/test/src/org/chromium/cronet_test_apk/CronetTestActivity.java
+++ b/components/cronet/android/test/src/org/chromium/cronet_test_apk/CronetTestActivity.java
@@ -74,8 +74,6 @@ public class CronetTestActivity extends Activity {
             return;
         }
 
-        mHistogramManager = new HistogramManager();
-
         String skipInitString = getCommandLineArg(SKIP_FACTORY_INIT_KEY);
         if (skipInitString != null) {
             return;
@@ -121,6 +119,8 @@ public class CronetTestActivity extends Activity {
         mUrlRequestContext = UrlRequestContext.createContext(
                 getApplicationContext(), config);
 
+        mHistogramManager = HistogramManager.createHistogramManager();
+
         return HttpUrlRequestFactory.createFactory(getApplicationContext(),
                                                    config);
     }