Bug 1395418 Revert a previous patch that disabled std::thread on MinG…

…W because we didn't have it. Now we do. r=cpearce

MozReview-Commit-ID: 8ah0Unum1f2

--HG--
extra : rebase_source : 7054f778c0b8a2e4a831e379086243433cd219b8

media/gmp-clearkey/0.1/VideoDecoder.cpp

@@ -38,12 +38,7 @@ VideoDecoder::VideoDecoder(Host_8 *aHost)
 
   mDecoder = new WMFH264Decoder();
 
-  // MinGW Does not support std::thread (See #1349912)
-#ifdef __MINGW32__
-  uint32_t cores = 1u;
-#else
   uint32_t cores = std::max(1u, std::thread::hardware_concurrency());
-#endif
 
   HRESULT hr = mDecoder->Init(cores);
 }