skiaserve: Fix /cmd/N and /cmd/N[0|1], the response was never queued.

BUG=skia:
R=joshualitt@google.com, ethannicholas@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1686553006

Review URL: https://codereview.chromium.org/1686553006
diff --git a/tools/skiaserve/skiaserve.cpp b/tools/skiaserve/skiaserve.cpp
index e2e5450..6bc7cc7 100644
--- a/tools/skiaserve/skiaserve.cpp
+++ b/tools/skiaserve/skiaserve.cpp
@@ -143,6 +143,18 @@
     return MHD_YES;
 }
 
+// SendOK just sends an empty response with a 200 OK status code.
+static int SendOK(MHD_Connection* connection) {
+    const char* data = "";
+
+    MHD_Response* response = MHD_create_response_from_buffer(strlen(data),
+                                                             (void*)data,
+                                                             MHD_RESPMEM_PERSISTENT);
+    int ret = MHD_queue_response(connection, 200, response);
+    MHD_destroy_response(response);
+    return ret;
+}
+
 static int SendData(MHD_Connection* connection, const SkData* data, const char* type,
                     bool setContentDisposition = false, const char* dispositionString = nullptr) {
     MHD_Response* response = MHD_create_response_from_buffer(data->size(),
@@ -228,7 +240,7 @@
             int n;
             sscanf(commands[1].c_str(), "%d", &n);
             request->fDebugCanvas->deleteDrawCommandAt(n);
-            return MHD_YES;
+            return SendOK(connection);
         }
 
         // /cmd/N/[0|1]
@@ -237,7 +249,7 @@
             sscanf(commands[1].c_str(), "%d", &n);
             sscanf(commands[2].c_str(), "%d", &toggle);
             request->fDebugCanvas->toggleCommand(n, toggle);
-            return MHD_YES;
+            return SendOK(connection);
         }
 
         return MHD_NO;