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::ApplyPreferences() {
  WebPreferences prefs = render_view()->GetWebkitPreferences();
  ExportLayoutTestSpecificPreferences(prefs_, &prefs);
  render_view()->SetWebkitPreferences(prefs);
  Send(new ShellViewHostMsg_OverridePreferences(routing_id(), prefs));
}

void BlinkTestRunner::ApplyPreferences() {
  WebPreferences prefs = render_view()->GetWebkitPreferences();
  ExportLayoutTestSpecificPreferences(prefs_, &prefs);
  render_view()->SetWebkitPreferences(prefs);
  Send(new ShellViewHostMsg_OverridePreferences(routing_id(), prefs));
}
