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 RenderFrameImpl::SetPendingNavigationParams(
    scoped_ptr<NavigationParams> navigation_params) {
  pending_navigation_params_ = navigation_params.Pass();
}

void RenderFrameImpl::SetPendingNavigationParams(
    scoped_ptr<NavigationParams> navigation_params) {
  pending_navigation_params_ = navigation_params.Pass();
}
