Bug 797666: Add unsigned cast in assertion within TiledLayerBuffer::Update(), to fix build warning. r=BenWa

gfx/layers/TiledLayerBuffer.h

@@ -365,7 +365,10 @@ TiledLayerBuffer<Derived, Tile>::Update(const nsIntRegion& aNewValidRegion,
       int tileX = floor_div(x - newBufferOrigin.x, GetTileLength());
       int tileY = floor_div(y - newBufferOrigin.y, GetTileLength());
       int index = tileX * mRetainedHeight + tileY;
-      NS_ABORT_IF_FALSE(index >= 0 && index < newRetainedTiles.Length(), "index out of range");
+      NS_ABORT_IF_FALSE(index >= 0 &&
+                        static_cast<unsigned>(index) < newRetainedTiles.Length(),
+                        "index out of range");
+
       Tile newTile = newRetainedTiles[index];
       while (IsPlaceholder(newTile) && oldRetainedTiles.Length() > 0) {
         AsDerived().SwapTiles(newTile, oldRetainedTiles[oldRetainedTiles.Length()-1]);