Support desktop media pipeline on Android for GN build.

third_party/opus/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("//build/config/arm.gni")
 import("//testing/test.gni")
 
 # If fixed point implementation shall be used (otherwise float).
 use_opus_fixed_point = current_cpu == "arm" || current_cpu == "arm64"
 
 # If ARM optimizations shall be used to accelerate performance.
 use_opus_arm_optimization = current_cpu == "arm"
 
 # If OPUS Run Time CPU Detections (RTCD) shall be used.
 # Based on the conditions in celt/arm/armcpu.c:
 # defined(_MSC_VER) || defined(__linux__).
 use_opus_rtcd = current_cpu == "arm" && (is_win || is_android || is_linux)
 
 config("opus_config") {
   include_dirs = [ "src/include" ]
+
+  defines = []
+  if (use_opus_fixed_point) {
+    defines += [ "OPUS_FIXED_POINT" ]

[minyue, 2015/10/15 08:20:24]

The corresponding switch in gyp is configured this way:

https://codereview.chromium.org/1230793009/diff/240001/third_party/opus/opus.gyp

I think in GYP, OPUS_FIXED_POINT is to tell targets that depend on this Opus
build that this Opus is FIXED_PONT.

Is the proposed GN change equivalent? why having "defines = []"

[Tima Vaisburd, 2015/10/15 18:11:49]

Unfortunately I'm not an expert in GN either. Here is what Cookbook
https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/cookboo...
says about direct_dependent_settings:
"In GN, put the settings in a config (declared above your target), and then
reference that as a public config in your target" (there is an example there.

I tried to follow that document and make the settings that are equivalent to
GYP.
I believe in this case the config("opus_config") is the config that can use and
it is already made public on l.79
I believe this is the part of GN syntax. Without prior (empty) list the
following operator '+=' does not work. 

[minyue, 2015/10/15 18:32:15]

Thank you for pointing out the doc. That is quite clear. I think the define = []
is still unnecessary.

May you try
   if (use_opus_fixed_point) {
      defines = [ "OPUS_FIXED_POINT" ]

[Tima Vaisburd, 2015/10/15 18:51:50]

Yes, that seems to work too. Modified the way you proposed.

+  }
 }
 
 config("opus_test_config") {
   include_dirs = [
     "src/celt",
     "src/silk",
   ]
 
   if (is_win) {
     defines = [ "inline=__inline" ]
@@ skipping 223 matching lines @@
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     ":opus_test_config",
   ]
 
   deps = [
     ":opus",
   ]
 }