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::OnSessionHistory(
    const std::vector<int>& routing_ids,
    const std::vector<std::vector<PageState>>& session_histories,
    const std::vector<unsigned>& current_entry_indexes) {
  routing_ids_ = routing_ids;
  session_histories_ = session_histories;
  current_entry_indexes_ = current_entry_indexes;
  CaptureDump();
}

void BlinkTestRunner::OnSessionHistory(
    const std::vector<int>& routing_ids,
    const std::vector<std::vector<PageState>>& session_histories,
    const std::vector<unsigned>& current_entry_indexes) {
  routing_ids_ = routing_ids;
  session_histories_ = session_histories;
  current_entry_indexes_ = current_entry_indexes;
  CaptureDump();
}
