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This CL is adding a ScopedThreadPriority that save the current
thread priority and restore it when leaving the scope. This is
required since it is possible to run code outside of the Run(...)
scope with Background priority.

On Windows, this is causing shutdown hangs because the LoaderLock
can be held by a background thread while doing TLS destruction.


Without this fix, the DCHECK in the code run with Background priority
and fails:

[9888:28656:0617/210735.064:FATAL:thread_local_storage.cc(331)] Check failed: PlatformThread::GetCurrentThreadPriority() != ThreadPriority::BACKGROUND (0 vs. 0)
Backtrace:
        base::debug::CollectStackTrace [0x00007FF92445E7D2+18] (C:\src\chromium\src\base\debug\stack_trace_win.cc:284)
        base::debug::StackTrace::StackTrace [0x00007FF9243668F2+18] (C:\src\chromium\src\base\debug\stack_trace.cc:203)
        logging::LogMessage::~LogMessage [0x00007FF92437F094+148] (C:\src\chromium\src\base\logging.cc:605)
        logging::LogMessage::~LogMessage [0x00007FF92437FE40+16] (C:\src\chromium\src\base\logging.cc:598)
        base::internal::PlatformThreadLocalStorage::OnThreadExit [0x00007FF9244425FA+170] (C:\src\chromium\src\base\threading\thread_local_storage.cc:333)
        RtlActivateActivationContextUnsafeFast [0x00007FF96440B583+291]
        RtlActivateActivationContextUnsafeFast [0x00007FF96440B67F+543]
        LdrShutdownThread [0x00007FF964408B8C+348]
        RtlExitUserThread [0x00007FF9644436FE+62]
        BaseThreadInitThunk [0x00007FF96240403C+28]
        RtlUserThreadStart [0x00007FF964443691+33]

Bug: 1096203
Change-Id: Ibc011ac370dde4b6a872da6ac81938f52965a5a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2251199
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779932}