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This CL adds a switch allowing the choose which icon to use for the
search button in the bottom toolbar.

Bug: 826192
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I363822bd9fa94606bf5422b0664febc91b81e03e
Reviewed-on: https://chromium-review.googlesource.com/980935
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Eric Noyau <noyau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546099}