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

The following directories we don't use are removed:

as_is
packaging
source/layout
source/layoutex

Better plural formatting, updated locale data, the segmentation support
for Chinese, Japanese (our code was upstreamed) and Khmer/Lao are among
many improvements. EcmaScript I18N API also need this update to be compliant
to the spec.

Because codereview.chromium.org keeps giving me a 500 error when uploading
the CL, so this initial check-in is done directly with svn.

BUG=132145
TEST=None


git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu52@259715 4ff67af0-8c30-449e-8e8b-ad334ec8d88c