Freeze iOS kernel version in user-agent

Freeze the iOS kernel version in the user-agent string to be "15E148"
for iOS 11.3 and later for consistency with Safari and to reduce the
fingerprinting surface.

Likewise, reduce OS version granularity from three numbers to two
numbers for iOS 11.3 and later.

BUG=860229

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I0165ba4ae9dfd51308e26ebecba1a5dfba3df398
Reviewed-on: https://chromium-review.googlesource.com/1126105
Commit-Queue: Thiemo Nagel <tnagel@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574910}
1 file changed