Utf*Characterstream caches the data pointer of ExternalStrings through
ExternalStringStream, so lock the strings in ExternalStringStream.

Bug: chromium:877044
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I241caaf64e109b33e2f9982573e11c514410509c
Reviewed-on: https://chromium-review.googlesource.com/1194003
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55613}