[omnibox] Change SuggestResult and AutocompleteMatch answer to optional

The SuggestAnswer fields in SuggestResult and AutocompleteMatch are
currently unique_ptr's to the actual answer, so that we don't have to
populate them if not present. This requires us to have custom copy
constructor and assignment methods. This CL changes the fields to
base::Optional<>. This allows copying and assignment to work
implicitly.

# For cocoa...unittest.mm tweak
TBR=ellyjones@chromium.org

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I5a42375fa855976aa0fab996be28a5ff44213e02
Reviewed-on: https://chromium-review.googlesource.com/1181165
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Kevin Bailey <krb@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587356}