Brings defines for the GN build in line with GYP.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 4e8e540349bd175ff37b22ce15d71c3ba66a8067..0ad2539bd86bc1ae14b1b8a67669b7375a2b701a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -16,10 +16,13 @@ group("icu") {
 # 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") {
@@ -35,6 +38,17 @@ config("icu_config") {
 # 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 += [