Fix string race conditions in WebSharedWorkerImpl
Fix string race conditions in WebSharedWorkerImpl

In C++, temporary variables are allowed to live as long as the statement
they're created in is running. If you call callOnMainThread in the same
statement as String::isolatedCopy, the string might be used on the main
thread before the temporary returned by isolatedCopy is destroyed.

R=jochen@chromium.org
BUG=223955

Review URL: https://codereview.chromium.org/174243002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@167540 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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