Bug 1454978 - Move MOZ_ASSERT(mIsMapped) into mChangeMutex. r=bas

We must check mIsMapped inside mChangeMutex or else we could run into a race condition where
the source surface is subsequently mapped again before the function terminates. This was
being hit on try.

MozReview-Commit-ID: 1eot3DVW8VD

--HG--
extra : rebase_source : 19f2514d9e2412ee5fe7c99844b7f64f78a7425e

gfx/2d/SourceSurfaceSkia.cpp

@@ -170,18 +170,19 @@ SourceSurfaceSkia::Map(MapType, MappedSurface *aMappedSurface)
   aMappedSurface->mData = GetData();
   aMappedSurface->mStride = Stride();
   mIsMapped = !!aMappedSurface->mData;
+  bool isMapped = mIsMapped;
   if (!mIsMapped) {
     mChangeMutex.Unlock();
   }
-  return mIsMapped;
+  return isMapped;
 }
 
 void
 SourceSurfaceSkia::Unmap()
 {
-  mChangeMutex.Unlock();
   MOZ_ASSERT(mIsMapped);
   mIsMapped = false;
+  mChangeMutex.Unlock();
 }
 
 void