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