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This CL restrict the user agent restoration to iOS 13 and further as
iOS 12 has an issue getting the right User Agent.

Bug: 1070593
Change-Id: Ie4570efcb9931ccb8662511b16a1298570f0a297
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154189Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760057}