[base] Support ::GetThreadPriority() returning 3.

We have observed ::GetThreadPriority returning 3 after the thread
priority is set to NORMAL on Windows 7. This CL adds code to
correctly map 3 to ThreadPriority::NORMAL in
PlatformThread::GetCurrentThreadPriority(). From documentation,
that should only happen in a REALTIME_PRIORITY_CLASS process,
but since we can't get the thread and process priority atomically,
we can hardly add a robust DCHECK for that.

Bug: 1014659, 872820
Change-Id: If43b9d46fd766dd1d22e0efaeafe0b738ee57d4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898209
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712564}

base/threading/platform_thread_win.cc

@@ -33,6 +33,10 @@ namespace {
 // thread mode is enabled on Windows 7.
 constexpr int kWin7BackgroundThreadModePriority = 4;
 
+// Value sometimes returned by ::GetThreadPriority() after thread priority is
+// set to normal on Windows 7.
+constexpr int kWin7NormalPriority = 3;
+
 // The information on how to set the thread name comes from
 // a MSDN article: http://msdn2.microsoft.com/en-us/library/xcb2z8hs.aspx
 const DWORD kVCThreadNameException = 0x406D1388;
@@ -441,6 +445,9 @@ ThreadPriority PlatformThread::GetCurrentThreadPriority() {
     case kWin7BackgroundThreadModePriority:
       DCHECK_EQ(win::GetVersion(), win::Version::WIN7);
       return ThreadPriority::BACKGROUND;
+    case kWin7NormalPriority:
+      DCHECK_EQ(win::GetVersion(), win::Version::WIN7);
+      FALLTHROUGH;
     case THREAD_PRIORITY_NORMAL:
       return ThreadPriority::NORMAL;
     case THREAD_PRIORITY_ABOVE_NORMAL: