Fix pretty-printer for WTF::String

It broke with:

https://chromium-review.googlesource.com/c/chromium/src/+/2301589

Also, add my self to OWNERS, because I care.

Change-Id: Ibb344fc4f432b5d3815ea40ab8e121e9403fa513
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316946


Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791496}