bindings: Dictionary should use thread-specific emptyExceptionState()
bindings: Dictionary should use thread-specific emptyExceptionState()

emptyExceptionState() is used when a Dictionary is instanciated
without an ExceptionState. Since multiple threads (specifically,
workers) can use Dictionary, it should be thread-specific.

TEST=manually ran tsan
BUG=449905

Review URL: https://codereview.chromium.org/862593002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@188652 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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