Omnibox: SearchProvider: Fix Navsuggestions in Prevent-Inlining Mode

Precisely, navsuggestions that look like a user's input (trailing slash or not, omitted scheme/droppable prefix)--are effectively a what-you-typed suggestion--sometimes were previously erroneously marked as "not allowed to be the default match" when the user is backspacing (i.e., prevent_inline_autocompletion mode).

This causes some wonkiness, especially now that reorder mode is default.  See listed bug.

This fixes the trouble and added/corrects tests.

BUG=344585

Review URL: https://codereview.chromium.org/171753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252486 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252486