Update ICU to 58 part1

* Note that this CL will be followed by CLs with local changes.
  Until then, ICU should not be rolled in DEPS. See READ_THIS_FIRST
  for details.

* Adjust scripts/update.sh and scripts/data_files_to_preserve.txt
  - CLDR/ICU added ckb/ast locale data. Drop them from the list to preserve.
  - source/layout does not exist in 58.1 any more.

* Update the tree to ICU 58.1 from the upstream by running

* Update README.chromium and add READ_THIS_FIRST to warn about the
  status of the tree.