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::ResourceDispatcherHostCreated() {
  resource_dispatcher_host_delegate_.reset(
      base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kRunLayoutTest)
          ? new LayoutTestResourceDispatcherHostDelegate
          : new ShellResourceDispatcherHostDelegate);
  ResourceDispatcherHost::Get()->SetDelegate(
      resource_dispatcher_host_delegate_.get());
}

void ShellContentBrowserClient::ResourceDispatcherHostCreated() {
  resource_dispatcher_host_delegate_.reset(
      base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kRunLayoutTest)
          ? new LayoutTestResourceDispatcherHostDelegate
          : new ShellResourceDispatcherHostDelegate);
  ResourceDispatcherHost::Get()->SetDelegate(
      resource_dispatcher_host_delegate_.get());
}
