[ChromeDriver] Update error code of ExecuteSetAlertText()

Make ExecuteSetAlertText() return kInvalidArgument according W3C standard

Bug: chromedriver:2003
Change-Id: Ibd65e8861f0a3f17bb21b54b0d031b88de7dc05c
Reviewed-on: https://chromium-review.googlesource.com/1226794Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Tatiana Buldina <buldina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591428}

chrome/test/chromedriver/alert_commands.cc

@@ -67,7 +67,7 @@ Status ExecuteSetAlertText(Session* session,
                            std::unique_ptr<base::Value>* value) {
   std::string text;
   if (!params.GetString("text", &text))
-    return Status(kUnknownError, "missing or invalid 'text'");
+    return Status(kInvalidArgument, "missing or invalid 'text'");
 
   JavaScriptDialogManager* dialog_manager =
       web_view->GetJavaScriptDialogManager();

docs/chromedriver_status.md

@@ -55,6 +55,6 @@ Below is a list of all WebDriver commands and their current support in ChromeDri
 | POST   | /session/{session id}/alert/dismiss                            | Dismiss Alert              | Complete           |
 | POST   | /session/{session id}/alert/accept                             | Accept Alert               | Complete           |
 | GET    | /session/{session id}/alert/text                               | Get Alert Text             | Complete           |
-| POST   | /session/{session id}/alert/text                               | Send Alert Text            | Partially Complete | [2003](https://bugs.chromium.org/p/chromedriver/issues/detail?id=2003)
+| POST   | /session/{session id}/alert/text                               | Send Alert Text            | Complete           |
 | GET    | /session/{session id}/screenshot                               | Take Screenshot            |                    |
 | GET    | /session/{session id}/element/{element id}/screenshot          | Take Element Screenshot    | Complete           |