Connect WebUSB client interface to the devices app

This provides a basic WebUSB client interface in
content/renderer. Most of the interface is unimplemented,
but this CL hooks up navigator.usb.getDevices() to the
browser's Mojo devices app to enumerate available USB
devices.

BUG=492204

Review URL: https://codereview.chromium.org/1293253002

Cr-Commit-Position: refs/heads/master@{#344881}
void RenderViewTest::SendWebMouseEvent(
    const blink::WebMouseEvent& mouse_event) {
  RenderViewImpl* impl = static_cast<RenderViewImpl*>(view_);
  impl->OnMessageReceived(
      InputMsg_HandleInputEvent(0, &mouse_event, ui::LatencyInfo(), false));
}

void RenderViewTest::SendWebMouseEvent(
    const blink::WebMouseEvent& mouse_event) {
  RenderViewImpl* impl = static_cast<RenderViewImpl*>(view_);
  impl->OnMessageReceived(
      InputMsg_HandleInputEvent(0, &mouse_event, ui::LatencyInfo(), false));
}
