DevTools: [SSP] live-update property value when it's selected via mouse.

There was a left-over condition in the TextPrompt which has never being
satisfied. As a result, the WebInspector.TextPrompt.Events.ItemAccepted
event has never being fired.

BUG=398224

Review URL: https://codereview.chromium.org/477593005

git-svn-id: svn://svn.chromium.org/blink/trunk@180453 bbb929c8-8fbe-4397-9dbb-9b2b20218538