[iOS] Add accessibility labels to text zoom buttons

Because the buttons show images, VoiceOver was reading the image
filename. There should be a real label instead.

Bug: 1028938
Change-Id: I3fdcb3e325460616d935cc594f4a7489ebbb033f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078474
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745099}
2 files changed