event_sender: Keep keyIdentifier in keypress event

Blink's actual implementation preserves keyIdentifier='Enter'
when Enter is pressed. To make sure that the test framework
behaves like the actual implementation, this CL makes sure that
keyIdentifier is set to 'Enter' when the Enter key is simulated.

BUG=423270

Review URL: https://codereview.chromium.org/649343002

Cr-Commit-Position: refs/heads/master@{#302403}

content/shell/renderer/test_runner/event_sender.cc

@@ -1361,7 +1361,11 @@ void EventSender::KeyDown(const std::string& code_str,
   if (generate_char) {
     WebKeyboardEvent event_char = event_up;
     event_char.type = WebInputEvent::Char;
-    event_char.keyIdentifier[0] = '\0';
+    // keyIdentifier is an empty string, unless the Enter key was pressed.
+    // This behavior is not standard (keyIdentifier itself is not even a
+    // standard any more), but it matches the actual behavior in Blink.
+    if (code != ui::VKEY_RETURN)
+      event_char.keyIdentifier[0] = '\0';
     view_->handleInputEvent(event_char);
   }