[ChromeDriver] Don't crash with unknown status

In W3C mode, when execution of a command results in an unknown status,
ChromeDriver was crashing due to an uninitialized variable. Now fixed.

Bug: chromedriver:3007
Change-Id: I662dfdf9d4d6b5de3396f90258e1df3d1cfebae5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693168
Commit-Queue: John Chen <johnchen@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Auto-Submit: John Chen <johnchen@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675788}

chrome/test/chromedriver/server/http_handler.cc

@@ -1161,6 +1161,12 @@ HttpHandler::PrepareStandardResponse(
       response.reset(
           new net::HttpServerResponseInfo(net::HTTP_INTERNAL_SERVER_ERROR));
       break;
+
+    default:
+      DCHECK(false);
+      response.reset(
+          new net::HttpServerResponseInfo(net::HTTP_INTERNAL_SERVER_ERROR));
+      break;
   }
 
   if (!value)