GitLab

- Change namespace from search_box to ash.
- Rename some constants to add prefix kSearchBox
- Eliminate SEARCH_BOX_EXPORT macros. They aren't needed because
  search box is just a source_set.

Bug: 1131641
Test: compiles, ash_unittests
Change-Id: I55a4706bc57de90a823663a6f42f4554c09577b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2430043
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810461}