Wait for ninja proto processing to finish when exiting

Wait for the ninja proto processing goroutine to notice the fifo
has closed and exit before continuing.

Test: m nothing
Change-Id: I8cf5f3b8bf6a91496c6d2bbbd3e811eb7f0c9d21

ui/build/ninja.go

@@ -31,7 +31,8 @@ func runNinja(ctx Context, config Config) {
 	defer ctx.EndTrace()
 
 	fifo := filepath.Join(config.OutDir(), ".ninja_fifo")
-	status.NinjaReader(ctx, ctx.Status.StartTool(), fifo)
+	nr := status.NewNinjaReader(ctx, ctx.Status.StartTool(), fifo)
+	defer nr.Close()
 
 	executable := config.PrebuiltBuildTool("ninja")
 	args := []string{