GN: Fix compile errors with os==chromeos mojo/public build

build/config/arm.gni

 # 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.
 
 if (cpu_arch == "arm") {
   declare_args() {
     # Version of the ARM processor when compiling on ARM. Ignored on non-ARM
     # platforms.
     arm_version = 7
 
     # The ARM floating point mode. This is either the string "hard", "soft", or
     # "softfp". An empty string means to use the default one for the
     # arm_version.
     arm_float_abi = ""
+
+    # The ARM variant-specific tuning mode. This will be a string like "armv6"
+    # or "cortex-a15". An empty string means to use the default for the
+    # arm_version.
+    arm_tune = ""
+
+    # Whether to use the neon FPU instruction set or not.
+    arm_use_neon = true
   }
 
   assert(arm_float_abi == "" ||
          arm_float_abi == "hard" ||
          arm_float_abi == "soft" ||
          arm_float_abi == "softfp")
 
   if (is_android) {
     arm_use_neon = false
-    arm_optionally_use_neon = true
-  } else {
-    arm_use_neon = true
-    arm_optionally_use_neon = true
   }
+  arm_optionally_use_neon = true
 
   if (arm_version == 6) {
     arm_arch = "armv6"
-    arm_tune = ""
+    if (arm_tune != "") {
+      arm_tune = ""
+    }
     if (arm_float_abi == "") {
       arm_float_abi = "softfp"
     }
     arm_fpu = "vfp"
     # Thumb is a reduced instruction set available on some ARM processors that
     # has increased code density.
     arm_use_thumb = false
 
   } else if (arm_version == 7) {
     arm_arch = "armv7-a"
-    arm_tune = "generic-armv7-a"
+    if (arm_tune == "") {
+      arm_tune = "generic-armv7-a"
+    }
+
     if (arm_float_abi == "") {
       arm_float_abi = "softfp"
     }
+
     arm_use_thumb = true
 
     if (arm_use_neon) {
       arm_fpu = "neon"
     } else {
       arm_fpu = "vfpv3-d16"
     }
   }
 }