Add chrome.searchEnginesPrivate API for setting and getting the default search engines.

Also add the first parts of the Search settings UI in MD-settings, namely the 'default search engine' dropdown. Follow-up CLs will fill in the remaining functionality.

(Disabling presubmit because it requires a manual confirmation that permission_message.h looks ok. I confirmed this.)

BUG=479359
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1096143003

Cr-Commit-Position: refs/heads/master@{#326844}
34 files changed