Add icu.isolate

icu.isolate add either icudtl.dat or icudt.dll to |track...| depending on
icu_use_data_file_flag and OS.

icu.isolate will be included by *isolate files that currently list icudt.dll on
Windows.

This is a prerequisite for the following CLs to use icudtl.dat :

Windows : https://codereview.chromium.org/99473012
Linux : https://codereview.chromium.org/102413007
Mac : https://codereview.chromium.org/109013004

It's also a prerequisite for the following CLs to refer to icu.isolate :

https://codereview.chromium.org/124143002/

BUG=72633
TEST=Once rolled in, *swam testbots work fine.
R=mark@chromium.org, maruel@chromium.org

Review URL: https://codereview.chromium.org/99073008

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu46@243376 4ff67af0-8c30-449e-8e8b-ad334ec8d88c