DevTools: Initialize the "Go to member" dialog query to the current selection

This patch automatically initializes the "Go to member" query for JS and CSS
to the current text selection when opening the dialog.

R=aandrey, vsevik
BUG=392198

Review URL: https://codereview.chromium.org/374893003

git-svn-id: svn://svn.chromium.org/blink/trunk@177758 bbb929c8-8fbe-4397-9dbb-9b2b20218538