Bug 1529466 - Make `TextInputHandler::HandleCommand()` dispatch `keydown`/`keypress` events for emulating `Tab` key when `insertTab:` or `intertBacktab:` command is received r=m_kato

`TextInputHandler::HandleCommand()` dispatches `Tab` key events only when
it receives `insertTab:` or `intertBacktab:` command which are caused by
actual `Tab` key press.

However, these commands could be generated by IME or the system shortcut
keys are customized oddly.

This patch makes the method support such situation with dispatching fake
`keydown` and `keypress` events to emulate `Tab` key press for
mainly focus navigation.

Differential Revision: https://phabricator.services.mozilla.com/D20804

--HG--
extra : moz-landing-system : lando

widget/cocoa/TextInputHandler.mm

@@ -2642,6 +2642,16 @@ bool TextInputHandler::HandleCommand(Command aCommand) {
         }
         break;
       }
+      case CommandInsertTab:
+      case CommandInsertBacktab:
+        nsCocoaUtils::InitInputEvent(keypressEvent, keyEvent);
+        keypressEvent.mKeyCode = NS_VK_TAB;
+        keypressEvent.mKeyNameIndex = KEY_NAME_INDEX_Tab;
+        keypressEvent.mModifiers &= ~(MODIFIER_CONTROL | MODIFIER_ALT | MODIFIER_META);
+        if (aCommand == CommandInsertBacktab) {
+          keypressEvent.mModifiers |= MODIFIER_SHIFT;
+        }
+        break;
       case CommandDeleteCharBackward:
       case CommandDeleteToBeginningOfLine:
       case CommandDeleteWordBackward: {