REPRO STEPS:

Use ui_test_utils to simulate start of a drag and drop:

1. // Simulate mouse move to the middle of an image / frame:
   ui_test_utils::SendMouseMoveSync(kMiddleOfLeftFrame);

2. // Simulate mouse down:
   ui_test_utils::SendMouseEventsSync(ui_controls::LEFT,
                                      ui_controls::DOWN);
3. // Simulate dragging the mouse:
   ui_test_utils::SendMouseMoveSync(kMiddleOfLeftFrame +
                                    gfx::Vector2d(10, 10)))

EXPECTED BEHAVIOR:

- Step #3 triggers sending to renderer a WebInputEvent with
  button field set to WebPointerProperties::Button::Left

- Drag starts.

The expected and desired behavior above is what happens with Aura, but without Ozone:
    $ cat out/cros/args.gn 
    dcheck_always_on = true
    is_component_build = true
    is_debug = false
    use_goma = true
    enable_nacl = false
    target_os = "chromeos"


ACTUAL BEHAVIOR:

- Step #3 triggers sending to renderer a WebInputEvent with
  button field set to WebPointerProperties::Button::NoButton

- Drag doesn't start, because WebInputEventResult MouseEventManager::handleMouseDraggedEvent explicitly checks whether a mouse button was held down:

    WebInputEventResult MouseEventManager::handleMouseDraggedEvent(
        const MouseEventWithHitTestResults& event) {
    ...
      if (event.event().pointerProperties().button !=
          WebPointerProperties::Button::Left)
        m_mousePressed = false;
    
      if (!m_mousePressed)
        return WebInputEventResult::NotHandled;

The unexpected behavior above is what happens with Ozone:
    $ cat out/ozone/args.gn 
    dcheck_always_on = true
    is_component_build = true
    is_debug = false
    use_goma = true
    enable_nacl = false
    target_os = "chromeos"
    use_ozone = true