Allow raw string literals and add a sample usage. This was tentatively approved on the discussion thread two months ago. BUG=none TEST=none Review-Url: https://codereview.chromium.org/2470643002 Cr-Original-Commit-Position: refs/heads/master@{#429402} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: bf8b8103771747ed31d3af4e924de7cd27607deb