Add CrossThreadCopier and isolatedCopy() to SourceLocation.

We have to make isolatedCopy of every WTF::String when passing
SourceLocation between threads.

This fixes data race introduced by https://codereview.chromium.org/2010603002.

BUG=615712

Review-Url: https://codereview.chromium.org/2024993003
Cr-Commit-Position: refs/heads/master@{#397137}
7 files changed