cocoa: announce omnibox autocompletes

This change:
1) Adds AutocompleteMatchType::ToAccessibilityLabel, which turns a match type
   and match text into a descriptive string for an accessibility label;
2) Adds descriptive string templates for most types of autocomplete match;
3) Has OmniboxViewMac announce when the temporary text changes, instead of just
   when the autocomplete text changes.

Bug: 696233
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: If1df0e49e212810ad7d3bc49c0b5d262e107f7c7
Reviewed-on: https://chromium-review.googlesource.com/728900
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Rohit Rao (ping after 24h) <rohitrao@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514452}
14 files changed