Add dependency to ICU 4.6

Add dependency to ICU 4.6

We use the patched version from the chromium project, as it already has
a gyp file and allows for using the system icu instead of the bundled
version if desired.

This is in preparation of bringing in v8-i18n.

R=jkummerow@chromium.org
BUG=v8:2745
Committed: http://code.google.com/p/v8/source/detail?r=15402

[jochen (gone - plz use gerrit), 2013-06-27 11:24:13]

I still need to patch the icu.gyp file, so this doesn't compile yet, however,
the v8 side is done, so please have a look

[jochen (gone - plz use gerrit), 2013-06-29 19:40:40]

rebased. ptal.

[Jakob Kummerow, 2013-07-01 07:51:38]

LGTM, I'll land.

[Jakob Kummerow, 2013-07-01 08:32:07]

On second thought, let's skip the dependency on ICU in all.gyp for now. I'm
making that change locally before landing.

[Jakob Kummerow, 2013-07-01 08:32:48]

Committed patchset #3 manually as r15402 (presubmit successful).