Fix OmahaRequest unittests in Brillo.

Moved lsb_release related tests to ImagePropertiesTest.

Bug: 26955860
Test: GTEST_FILTER="Omaha*" ./update_engine_unittests
Test: cros_workon_make update_engine --test

Change-Id: I124c9374e7556b04c65bdac80e1b89064dee4008
diff --git a/update_engine.gyp b/update_engine.gyp
index 40d6314..5f24a83 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -506,6 +506,7 @@
             'connection_manager_unittest.cc',
             'fake_shill_proxy.cc',
             'fake_system_state.cc',
+            'image_properties_chromeos_unittest.cc',
             'metrics_utils_unittest.cc',
             'omaha_request_action_unittest.cc',
             'omaha_request_params_unittest.cc',