Add ios/icudtl.dat and ios/patch_locale.sh.

Update README.chromium and BUILD.gn accordingly.
Update scripts/copy_data.sh to take (ios|common|android).

At the moment, iOS data is almost identical to that of Android, but in the future
more cuts may be made (e.g. dictionary data for breakiterator).

Bug: 718955
TEST: iOS Chrome works as before.
Review-Url: https://codereview.chromium.org/2743123002 .