This file has the relevant components from ICU copied to handle basic
UTF8/16/32 conversions. Components are copied from utf.h utf8.h utf16.h and
utf_impl.c

The main change is that U_/U8_/U16_ prefixes have been replaced with
CBU_/CBU8_/CBU16_ (for "Chrome Base") to avoid confusion with the "real" ICU
macros should ICU be in use on the system. For the same reason, the functions
and types have been put in the "base_icu" namespace.