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-Original-Commit-Position: refs/heads/master@{#397137}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 48e31ee8f0d774b8674b90a7167e648ada24292c
2 files changed