DevTools: do not trigger autocomplete on deletion

Currently, deletion of a single letter in the end of the
word will trigger autocompletion (if any). As this is considered
disturbing, this patch switches off autocompletion summoning for
this case.

R=vsevik, pfeldman

Review URL: https://codereview.chromium.org/399193002

git-svn-id: svn://svn.chromium.org/blink/trunk@178427 bbb929c8-8fbe-4397-9dbb-9b2b20218538