Thread::Stop() must be called before any subclass's destructor completes.
Update Thread documentation, fix all subclasses that had a problem.

This is to avoid a data racing between the destructor modifying the vtable, and
Thread::PreRun calling virtual method Run at the same time.

For example:
[ RUN      ] FileMediaEngineTest.TestGetCapabilities
==================
WARNING: ThreadSanitizer: data race on vptr (ctor/dtor vs virtual call) (pid=2967)
  Read of size 8 at 0x7d480000bd00 by thread T1:
    #0 talk_base::Thread::PreRun(void*) /mnt/data/b/build/slave/Linux_Tsan_v2/build/src/out/Release/../../talk/base/thread.cc:353 (libjingle_media_unittest+0x000000234da8)

  Previous write of size 8 at 0x7d480000bd00 by main thread:
    #0 talk_base::Thread::~Thread() /mnt/data/b/build/slave/Linux_Tsan_v2/build/src/out/Release/../../talk/base/thread.cc:158 (libjingle_media_unittest+0x00000023478c)
    #1 ~RtpSenderReceiver /mnt/data/b/build/slave/Linux_Tsan_v2/build/src/out/Release/../../talk/media/base/filemediaengine.cc:122 (libjingle_media_unittest+0x0000001b551f)
    ...

RISK=P2
TESTED=try bots and tsan
BUG=2078,2080
R=fischman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2428004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/talk@4999 4adac7df-926f-26a2-2b94-8c16560cd09d
16 files changed