[iOS] Cleanup Search Widget, use shared utils

There is now a shared class in ios/chrome/common (somewhere extensions
can access) that does the same thing as CreateSameConstraints. We should
use that instead.

Also, we can get rid of unnecessary @synthesize and method prototypes.

Bug: 938317
Change-Id: I54b7a3b855015c377ebdbb8c0be2d0e47698a811
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503417
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638064}
9 files changed