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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=184263

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

git-svn-id: svn://svn.chromium.org/blink/trunk@184512 bbb929c8-8fbe-4397-9dbb-9b2b20218538