Bug 1066132: Fix SourceSurfaceD2D1::DrawTargetWillChange() to not null out mDrawTarget itself. r=jrmuizel

gfx/2d/SourceSurfaceD2D1.cpp

@@ -95,7 +95,6 @@ SourceSurfaceD2D1::DrawTargetWillChange()
   mImage = mRealizedBitmap;
 
   DrawTargetD2D1::mVRAMUsageSS += mSize.width * mSize.height * BytesPerPixel(mFormat);
-  mDrawTarget = nullptr;
 
   // We now no longer depend on the source surface content remaining the same.
   MarkIndependent();