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

components/cronet.gypi

Index: components/cronet.gypi
diff --git a/components/cronet.gypi b/components/cronet.gypi
index 054d872335ba797c15e5969251a2101460f68e72..dc29726c632844f2683726729adf36f6e78a0880 100644
--- a/components/cronet.gypi
+++ b/components/cronet.gypi
@@ -11,11 +11,11 @@
           'target_name': 'cronet_jni_headers',
           'type': 'none',
           'sources': [
+            'cronet/android/java/src/org/chromium/net/CronetHistogramManager.java',
             'cronet/android/java/src/org/chromium/net/CronetUrlRequest.java',
             'cronet/android/java/src/org/chromium/net/CronetUrlRequestContext.java',
             'cronet/android/java/src/org/chromium/net/ChromiumUrlRequest.java',
             'cronet/android/java/src/org/chromium/net/ChromiumUrlRequestContext.java',
-            'cronet/android/java/src/org/chromium/net/HistogramManager.java',
           ],
           'variables': {
             'jni_gen_package': 'cronet',
@@ -122,6 +122,8 @@
             'cronet/android/chromium_url_request_priority_list.h',
             'cronet/android/chromium_url_request_context.cc',
             'cronet/android/chromium_url_request_context.h',
+            'cronet/android/cronet_histogram_manager.cc',
+            'cronet/android/cronet_histogram_manager.h',
             'cronet/android/cronet_loader.cc',
             'cronet/android/cronet_loader.h',
             'cronet/android/cronet_url_request.cc',
@@ -132,8 +134,6 @@
             'cronet/android/cronet_url_request_context.h',
             'cronet/android/cronet_url_request_context_adapter.cc',
             'cronet/android/cronet_url_request_context_adapter.h',
-            'cronet/android/histogram_manager.cc',
-            'cronet/android/histogram_manager.h',
             'cronet/android/url_request_adapter.cc',
             'cronet/android/url_request_adapter.h',
             'cronet/android/url_request_context_adapter.cc',
@@ -194,6 +194,7 @@
             'javac_includes': [
               '**/ChunkedWritableByteChannel.java',
               '**/ExtendedResponseInfo.java',
+              '**/HistogramManager.java',
               '**/HttpUrlConnection*.java',
               '**/HttpUrlRequest*.java',
               '**/ResponseInfo.java',
@@ -230,11 +231,11 @@
               '**/ChromiumUrlRequestError.java',
               '**/ChromiumUrlRequestFactory.java',
               '**/ChromiumUrlRequestPriority.java',
+              '**/CronetHistogramManager.java',
               '**/CronetResponseInfo.java',
               '**/CronetUrlRequest.java',
               '**/CronetUrlRequestContext.java',
               '**/CronetUrlRequestFactory.java',
-              '**/HistogramManager.java',
               '**/RequestPriority.java',
               '**/urlconnection/CronetInputStream.java',
               '**/urlconnection/CronetHttpURLConnection.java',