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}
RendererMediaPlayerManager* RenderFrameImpl::GetMediaPlayerManager() {
  if (!media_player_manager_)
    media_player_manager_ = new RendererMediaPlayerManager(this);
  return media_player_manager_;
}

RendererMediaPlayerManager* RenderFrameImpl::GetMediaPlayerManager() {
  if (!media_player_manager_)
    media_player_manager_ = new RendererMediaPlayerManager(this);
  return media_player_manager_;
}
