[VK] Fix broken flags.

Follow up to r1341437. We should not remove the flag, but always set
it to true instead.

Once the flag has been removed from the extension code, we can remove
these flags in chromium too.

TBR=stevenjb@chromium.org

Bug: 856195
Change-Id: I396910c98289b1634123a16074e7f6c980baf1f1
Reviewed-on: https://chromium-review.googlesource.com/c/1345710Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609914}

chrome/browser/extensions/api/virtual_keyboard_private/chrome_virtual_keyboard_delegate.cc

@@ -364,6 +364,7 @@ void ChromeVirtualKeyboardDelegate::OnHasInputDevices(
       "fullscreenhandwriting",
       base::FeatureList::IsEnabled(
           features::kEnableFullscreenHandwritingVirtualKeyboard)));
+  features->AppendString(GenerateFeatureFlag("virtualkeyboardmdui", true));
   features->AppendString(GenerateFeatureFlag(
       "imeservice", base::FeatureList::IsEnabled(
                         chromeos::features::kImeServiceConnectable)));