Script streaming: fix a shutdown crash.
Script streaming: fix a shutdown crash.

The race was: main thread was in ~WebThreadImpl, waiting for the tasks to
finish, and the ScriptStreamerThread part of the thread object was already
deleted. Then the background thread tried to call
ScriptStreamerThread::shared()->taskDone().

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/805793002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@187147 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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