Include ICU headers in a way that will also work with use_system_icu=1

Include ICU headers in a way that will also work with use_system_icu=1

TEST=none
BUG=28294
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=56396

[Evan Martin, 2010-08-13 21:54:50]

I'm concerned that these forms of includes will accidentally end up using the
system ICU, and mixing headers will end up causing subtle corruption issues.  (I
just checked, and system ICU ends up in "/usr/include/unicode", so those paths
definitely can match it unless every person who ever uses these headers knows to
modify their include path.)  What can we do?

[Evan Martin, 2010-08-17 00:56:58]

We talked about this on IRC, and we found that Chrome is already doing the form
of #include I was worrying about all over the place.  So I think my worry is
overstated.

LGTM