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
  scripts/update.sh

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

BUG=637001
TEST=None
diff --git a/source/common/stringtriebuilder.cpp b/source/common/stringtriebuilder.cpp
index 109fcdc..075d7c4 100644
--- a/source/common/stringtriebuilder.cpp
+++ b/source/common/stringtriebuilder.cpp
@@ -1,3 +1,5 @@
+// Copyright (C) 2016 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
 /*
 *******************************************************************************
 *   Copyright (C) 2010-2012, International Business Machines