Cleanup: Remove an unnecessary AutocompleteMatch& calculation

Cleanup: Remove an unnecessary AutocompleteMatch& calculation from
OmniboxEditModel::OpenMatch().

I traced the origin of this weird code back to the days before OpenMatch() took
an AutocompleteMatch; back then, we constructed one so we could get at fields
like |fill_into_edit| that weren't provided to OpenMatch().  Now that we pass
this in, I believe the calculated match should always match the passed-in match.

BUG=none
TEST=none
R=sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=232847

[sky, 2013-11-01 18:31:47]

LGTM

[Peter Kasting, 2013-11-04 23:20:23]

FYI, I had to revert the "dynamically calculate ShortcutsBackend" code, because
it doesn't work when we haven't seen a pending navigation and instead call
OnSuccessfulNavigation() directly.

[Peter Kasting, 2013-11-05 00:21:57]

Committed patchset #2 manually as r232847 (presubmit successful).