1. Replace 'src/' with a fresh copy of ICU 54.1 from the upstream.
2. Remove unncessary files/directories
3. Update the source/header file lists in icu.gypi and icu.gyp
4. Update README.chromium to warn that the ICU head is not usable for
   building Chromium/Blink/v8,yet.

BUG=428145
TEST=None

Review URL: https://codereview.chromium.org/845603002