Use char16_t for UChar

This is going to be used along with https://crrev.com/c/1361705 .

Bug: 911896

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 261d43a..e65edce 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -51,12 +51,6 @@
   } else {
     defines += [ "ICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_STATIC" ]
   }
-
-  if (is_win) {
-    defines += [ "UCHAR_TYPE=wchar_t" ]
-  } else {
-    defines += [ "UCHAR_TYPE=uint16_t" ]
-  }
 }
 
 # Config used only by ICU code.

icu.gyp

diff --git a/icu.gyp b/icu.gyp
index bc88d8b..428ce88 100644
--- a/icu.gyp
+++ b/icu.gyp
@@ -40,14 +40,7 @@
         ],
       }],
       ['OS=="win"', {
-        'defines': [
-          'UCHAR_TYPE=wchar_t',
-        ],
 	'cflags': [ '/utf-8' ],
-      },{
-        'defines': [
-          'UCHAR_TYPE=uint16_t',
-        ],
       }],
       ['(OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris" \
          or OS=="netbsd" or OS=="mac" or OS=="android" or OS=="qnx") and \