[Autofill/Autocomplete Feature] Substring matching instead of prefix matching.

With present implementation autofill/autocomplete suggestions are matched for
prefix for what user has typed.

This patch brings in suggestions with substring matching (i.e. substring
matching to the beginning of suggestion tokens) instead of prefix matching.
This patch keeps the feature behind a command line switch
--enable-suggestions-with-substring-match.

BUG=77194

Review URL: https://codereview.chromium.org/962673004

Cr-Commit-Position: refs/heads/master@{#338677}
25 files changed