See https://docs.google.com/document/d/1BX8V3ciUs8fV-x8dBI6rdz5_2dAmewgrUxIcWILXvGA/edit#heading=h.xqucj3afqd2g for details.

Today, when input arrives in the renderer while in single threaded mode (i.e. LayoutTests) the events are routed directly into Blink. This entirely circumvents InputHandlerProxy and the various queues, coalescing, and routing logic that's used in production.

To test the real-world logic, we've introduced several virtual/threaded test suites that run with --enable-threaded-compositing. However, the threaded tests are flaky and threaded test suites in layout tests are generally discouraged.

This bug tracks making the renderer use the same code path for input handling regardless of whether a real compositor thread exists. We can then remove the virtual/threaded suites used for input code coverage.