Rename metrics.cc to metrics_reporter_omaha.cc

Also rename the mock_metrics.h to mock_metrics_reporter.h

Bug: 30989466
Test: unit tests pass
Change-Id: Ic2297b86c61c3e3db7159bc2b00b513d4a9410e3
diff --git a/update_engine.gyp b/update_engine.gyp
index 467000e..32c6553 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -269,7 +269,7 @@
         'hardware_chromeos.cc',
         'image_properties_chromeos.cc',
         'libcurl_http_fetcher.cc',
-        'metrics.cc',
+        'metrics_reporter_omaha.cc',
         'metrics_utils.cc',
         'omaha_request_action.cc',
         'omaha_request_params.cc',