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}
WebString BlinkTestRunner::RegisterIsolatedFileSystem(
    const blink::WebVector<blink::WebString>& absolute_filenames) {
  std::vector<base::FilePath> files;
  for (size_t i = 0; i < absolute_filenames.size(); ++i)
    files.push_back(base::FilePath::FromUTF16Unsafe(absolute_filenames[i]));
  std::string filesystem_id;
  Send(new LayoutTestHostMsg_RegisterIsolatedFileSystem(
      routing_id(), files, &filesystem_id));
  return WebString::fromUTF8(filesystem_id);
}

WebString BlinkTestRunner::RegisterIsolatedFileSystem(
    const blink::WebVector<blink::WebString>& absolute_filenames) {
  std::vector<base::FilePath> files;
  for (size_t i = 0; i < absolute_filenames.size(); ++i)
    files.push_back(base::FilePath::FromUTF16Unsafe(absolute_filenames[i]));
  std::string filesystem_id;
  Send(new LayoutTestHostMsg_RegisterIsolatedFileSystem(
      routing_id(), files, &filesystem_id));
  return WebString::fromUTF8(filesystem_id);
}
