Don't assert about borrowed DT refcounts when async painting. (bug 1381393, r=mattwoodrow)

gfx/layers/client/TextureClient.cpp

@@ -509,7 +509,7 @@ TextureClient::Lock(OpenMode aMode)
 
   auto format = GetFormat();
   if (mIsLocked && CanExposeDrawTarget() &&
-      aMode == OpenMode::OPEN_READ_WRITE &&
+      (aMode & OpenMode::OPEN_READ_WRITE) == OpenMode::OPEN_READ_WRITE &&
       NS_IsMainThread() &&
       // the formats that we apparently expect, in the cairo backend. Any other
       // format will trigger an assertion in GfxFormatToCairoFormat.
@@ -557,7 +557,8 @@ TextureClient::Unlock()
     mBorrowedDrawTarget->DetachAllSnapshots();
     // If this assertion is hit, it means something is holding a strong reference
     // to our DrawTarget externally, which is not allowed.
-    MOZ_ASSERT(mBorrowedDrawTarget->refCount() <= mExpectedDtRefs);
+    MOZ_ASSERT_IF(!(mOpenMode & OpenMode::OPEN_ASYNC_WRITE),
+                  mBorrowedDrawTarget->refCount() <= mExpectedDtRefs);
 
     mBorrowedDrawTarget = nullptr;
   }