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 BlinkTestRunner::ShowDevTools(const std::string& settings,
                                   const std::string& frontend_url) {
  Send(new ShellViewHostMsg_ShowDevTools(
      routing_id(), settings, frontend_url));
}

void BlinkTestRunner::ShowDevTools(const std::string& settings,
                                   const std::string& frontend_url) {
  Send(new ShellViewHostMsg_ShowDevTools(
      routing_id(), settings, frontend_url));
}
