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Renames the existing flag, kAutofillUpstreamUseGooglePayOnAndroidBranding,
to kAutofillUpstreamUseGooglePayBrandingOnMobile as it also controls GPay
branding in iOS.

Adds an entry to the about flags page in iOS in order to control the flag.

Bug:841758

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I40d961f0106e14f37c5091ffbda7ac72620dd43c
Reviewed-on: https://chromium-review.googlesource.com/1053430
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557573}