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::LoadHTML(const char* html) {
  std::string url_str = "data:text/html;charset=utf-8,";
  url_str.append(html);
  GURL url(url_str);
  WebURLRequest request(url);
  request.setCheckForBrowserSideNavigation(false);
  GetMainFrame()->loadRequest(request);
  FrameLoadWaiter(view_->GetMainRenderFrame()).Wait();
}

void RenderViewTest::LoadHTML(const char* html) {
  std::string url_str = "data:text/html;charset=utf-8,";
  url_str.append(html);
  GURL url(url_str);
  WebURLRequest request(url);
  request.setCheckForBrowserSideNavigation(false);
  GetMainFrame()->loadRequest(request);
  FrameLoadWaiter(view_->GetMainRenderFrame()).Wait();
}
