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Wraps omnibox textfield into a container that positions the textfield
inside of itself and clips its bounds so that the optimal clipping is
used. The clipping logic replicates that of the pre-iOS 11 solution
with UITextField overrides that are now broken in iOS 11.
To mask the clipped ends of the omnibox, a ClippingMaskView is used
as a maskView of the clipping view.
All of this is behind kClippingTextfield flag that is enabled by
default.

Bug: 749788
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ia03649618397d19990d39914fe1fb1b152b2760d
Reviewed-on: https://chromium-review.googlesource.com/779191
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525322}