More error logging for requestAutocomplete.

http://www.whatwg.org/specs/web-apps/current-work/#dom-form-requestautocomplete now says:

"Note: User agents are encouraged to log the precise cause in a developer console, to aid debugging."

BUG=361216

Review URL: https://codereview.chromium.org/234113003

git-svn-id: svn://svn.chromium.org/blink/trunk@171582 bbb929c8-8fbe-4397-9dbb-9b2b20218538