Make real classes for BenchmarkRunnerClient, BenchmarkController and SectionsManager.

These are subclassed in the debug-runner.

Move the graph-related code to a new GraphController class.

Fix DeveloperResultsTable's _addTest() override to correctly add the rows
under `tbody`.

Share a bit more code between BenchmarkController and DebugBenchmarkController.

Remove the window-scope benchmarkRunnerClient; keep it local to BenchmarkController.