[Webapp Refactor] Implement Unittest for ClientSession.

Summary of changes:
1. Introduces a new class MockConnection for mocking the components
   required for a connection.
2. Implements unit tests for ClientSession.
3. Fixes bugs found by the unit tests.
4. Cleans up remoting.MockClientPlugin

BUG=478331

Review URL: https://codereview.chromium.org/1095063003

Cr-Commit-Position: refs/heads/master@{#326427}
diff --git a/remoting/remoting_webapp_files.gypi b/remoting/remoting_webapp_files.gypi
index c4274743..2e98b04 100644
--- a/remoting/remoting_webapp_files.gypi
+++ b/remoting/remoting_webapp_files.gypi
@@ -72,6 +72,7 @@
       'webapp/base/js/ipc_unittest.js',
       'webapp/base/js/protocol_extension_manager_unittest.js',
       'webapp/crd/js/apps_v2_migration_unittest.js',
+      'webapp/crd/js/client_session_unittest.js',
       'webapp/crd/js/desktop_viewport_unittest.js',
       'webapp/crd/js/client_session_factory_unittest.js',
       'webapp/crd/js/dns_blackhole_checker_unittest.js',