Move CallbackCounter from ios/chrome/browser to base/ios

This is more suitable place for it, as it will be used from different layers.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Iaa3a11f53730216d7c3794fc5cd771fd936c17be
Reviewed-on: https://chromium-review.googlesource.com/748601
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513265}

• base/BUILD.gn[diff]
• base/ios/callback_counter.h[Renamed from ios/chrome/browser/callback_counter.h - diff]
• base/ios/callback_counter.mm[Renamed from ios/chrome/browser/callback_counter.mm - diff]
• base/ios/callback_counter_unittest.mm[Renamed from ios/chrome/browser/callback_counter_unittest.mm - diff]
• ios/chrome/app/BUILD.gn[diff]
• ios/chrome/app/main_controller.mm[diff]
• ios/chrome/app/startup/BUILD.gn[diff]
• ios/chrome/browser/BUILD.gn[diff]
• ios/chrome/browser/browsing_data/BUILD.gn[diff]
• ios/chrome/browser/browsing_data/browsing_data_removal_controller.mm[diff]
• ios/chrome/test/app/BUILD.gn[diff]
• ios/chrome/test/app/history_test_util.mm[diff]
• ios/clean/chrome/app/BUILD.gn[diff]
• ios/clean/chrome/app/steps/BUILD.gn[diff]