Without a Linux-ish define for Fuchsia, neither U_WCHAR_IS_UTF16 nor
U_WCHAR_IS_UTF32 are defined, resulting in warnings in ICU code when
compiled on Fuchsia. Filed upstream at
http://www.icu-project.org/trac/ticket/13202.

BUG=706592
R=jshin@chromium.org

Review-Url: https://codereview.chromium.org/2885143008 .