Bug 1259785: Do a proper flush when taking a snapshot so our dependent targets and command lists get appropriately cleared. r=jrmuizel

MozReview-Commit-ID: D49eFNRGD2V

--HG--
extra : rebase_source : 79bc34d1822396e87eedb5913ac56ecbf1b65fa3

gfx/2d/DrawTargetD2D1.cpp

@@ -88,7 +88,7 @@ DrawTargetD2D1::Snapshot()
   }
   PopAllClips();
 
-  mDC->Flush();
+  Flush();
 
   mSnapshot = new SourceSurfaceD2D1(mBitmap, mDC, mFormat, mSize, this);