DevTools: [CodeMirrro] defer autocomplete controller initialization

Autocompletion controller intialization makes almost half of the time
of SourceFrame initialization.

In order to render editor as fast as possible, this patch defers
autocompletion initialization until the first need.

BUG=425550
R=vsevik

Review URL: https://codereview.chromium.org/670163002

git-svn-id: svn://svn.chromium.org/blink/trunk@184263 bbb929c8-8fbe-4397-9dbb-9b2b20218538