Omnibox: Reduce Bolding Flicker in SearchProvider

Do this by computing bolding at the time results are fetched from the
suggest server and keeping that same bolding regardless of what
keys the user presses later.

At the moment, this change only affects query suggestions, not
navsuggestions.

BUG=259486

Review URL: https://codereview.chromium.org/23164011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243998 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243998