Flattening the string upfront has performance benefits and we can
also simplify the implementation quite a bit.

Bug: v8:6780
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ic75bdfdf900d30e51e95cdf1cb8d09aab06332c6
Reviewed-on: https://chromium-review.googlesource.com/1014102Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52643}