Fix clang warnings in GN build.

build/secondary/third_party/icu/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.
 
 declare_args() {
   # Tells icu to load an external data file rather than rely on the icudata
   # being linked directly into the binary.
   #
   # This flag is a bit confusing. As of this writing, icu.gyp set the value to
   # 0 but common.gypi sets the value to 1 for most platforms (and the 1 takes
@@ skipping 28 matching lines @@
   }
 
   include_dirs = [
     "source/common",
     "source/i18n",
   ]
 }
 
 # Config used only by ICU code.
 config("icu_code") {
+  cflags = []
   if (is_win) {
     # Disable some compiler warnings.
-    cflags = [
+    cflags += [
       "/wd4005",  # Macro redefinition.
       "/wd4068",  # Unknown pragmas.
       "/wd4267",  # Conversion from size_t on 64-bits.
       "/wd4996",  # Deprecated functions.
     ]
   } else if (is_linux) {
-    cflags = [
+    cflags += [
       # Since ICU wants to internally use its own deprecated APIs, don't
       # complain about it.
       "-Wno-deprecated-declarations",
       "-Wno-unused-function",
     ]
-  } else if (is_clang) {
-    cflags = [
+  }
+  if (is_clang) {
+    cflags += [
       "-Wno-deprecated-declarations",
       "-Wno-logical-op-parentheses",
       "-Wno-tautological-compare",
       "-Wno-switch",
     ]
   }
 }
 
 component("icui18n") {
   sources = [
@@ skipping 431 matching lines @@
         sources = [ "mac/icudtl_dat.S" ]
       } else if (is_android) {
         sources = [ "android/icudtl_dat.S" ]
       } else {
         assert(false, "No icu data for this platform")
       }
       defines = [ "U_HIDE_DATA_SYMBOL" ]
     }
   }
 }