[ios] Update History Search UI to match Bookmarks

Show scrim when search controller is active but no query has been entered (see iOS Contacts app).
Allow cancelling by tapping the scrim.
Dedupped some constants as discussed in previous CLs.
Updated history search bar centering code to restore previous value on exit.

Bug: 882785
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Id4ba6dc704c70ce44334431f95c66c9ed133b3a0
Reviewed-on: https://chromium-review.googlesource.com/1219002
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591335}
10 files changed