Brings defines for the GN build in line with GYP.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/icu/config.gni")
 
 # Meta target that includes both icuuc and icui18n. Most targets want both.
 # You can depend on the individually if you need to.
 group("icu") {
   deps = [
     ":icui18n",
     ":icuuc",
   ]
 }
 
 # Shared config used by ICU and all dependents.
 config("icu_config") {
   defines = [
+    # Tell ICU to not insert |using namespace icu;| into its headers,
+    # so that chrome's source explicitly has to use |icu::|.
     "U_USING_ICU_NAMESPACE=0",
+
+    # We don't use ICU plugins and dyload is only necessary for them.
+    # NaCl-related builds also fail looking for dlfcn.h when it's enabled.
     "U_ENABLE_DYLOAD=0",
-    "UCONFIG_NO_NON_HTML5_CONVERSION=1",
-    "U_CHARSET_IS_UTF8=1",
   ]
 
   if (component_mode != "shared_library") {
     defines += [ "U_STATIC_IMPLEMENTATION" ]
   }
 
   include_dirs = [
     "source/common",
     "source/i18n",
   ]
 }
 
 # Config used only by ICU code.
 config("icu_code") {
   cflags = []
+  defines = [
+    "HAVE_DLOPEN=0",
+
+    # Only build encoding coverters and detectors necessary for HTML5.
+    "UCONFIG_NO_NON_HTML5_CONVERSION=1",
+
+    # No dependency on the default platform encoding.
+    # Will cut down the code size.
+    "U_CHARSET_IS_UTF8=1",
+  ]
+
   if (is_win) {
     # Disable some compiler warnings.
     cflags += [
       "/wd4005",  # Macro redefinition.
       "/wd4068",  # Unknown pragmas.
       "/wd4267",  # Conversion from size_t on 64-bits.
       "/wd4996",  # Deprecated functions.
     ]
   } else if (is_linux) {
     cflags += [
@@ skipping 499 matching lines @@
         sources = [
           "android/icudtl_dat.S",
         ]
       } else {
         assert(false, "No icu data for this platform")
       }
       defines = [ "U_HIDE_DATA_SYMBOL" ]
     }
   }
 }