make ScriptState::m_exception safe
make ScriptState::m_exception safe

BUG=

Review URL: https://chromiumcodereview.appspot.com/16331003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@151652 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs