content: Rename webkit_test_helpers.{cc,h} to blink_test_helpers.{cc,h}

Now that webkit/ is gone, we are preparing ourselves for the merge of
third_party/WebKit into //blink.

BUG=None
BUG=content_shell && content_unittests
R=avi@chromium.org

Review URL: https://codereview.chromium.org/1118183003

Cr-Commit-Position: refs/heads/master@{#328202}
void ShellContentBrowserClient::SelectClientCertificate(
    WebContents* web_contents,
    net::SSLCertRequestInfo* cert_request_info,
    scoped_ptr<ClientCertificateDelegate> delegate) {
  if (!select_client_certificate_callback_.is_null())
    select_client_certificate_callback_.Run();
}

void ShellContentBrowserClient::SelectClientCertificate(
    WebContents* web_contents,
    net::SSLCertRequestInfo* cert_request_info,
    scoped_ptr<ClientCertificateDelegate> delegate) {
  if (!select_client_certificate_callback_.is_null())
    select_client_certificate_callback_.Run();
}
