Make blink::SpaceSplitString thread-safe.

This fixes a race because both main thread and worker threads parse CSP
which uses blink::CSPDirectiveList which uses blink::SpaceSplitString.

Bug: 807838
Change-Id: Id1eafe17daf3d37f9c1b6fc64aaa18ea964c57bc
Reviewed-on: https://chromium-review.googlesource.com/c/1429819
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625142}
2 files changed