Image decoding: Read a multi-thread-accessed boolean within a lock

ThreadSafeDataTransport::m_allDataReceived is read and written on
different threads. Protect access with a lock.

BUG=418261

Review URL: https://codereview.chromium.org/622623002

git-svn-id: svn://svn.chromium.org/blink/trunk@183110 bbb929c8-8fbe-4397-9dbb-9b2b20218538