DevTools: Improve search experience

This patch makes several changes:
 - Find/Previous button in replace mode now work even before the search was started for the first time
 - It is now possible to start the search in backwards direction initially by pressing Shift+Enter in the input field.

BUG=356600
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/228293006

git-svn-id: svn://svn.chromium.org/blink/trunk@171125 bbb929c8-8fbe-4397-9dbb-9b2b20218538