Fix for Win GN Build.

This changes it to inherit common configuration, in order to LOG() macro
take effect (hopefully).

This should fix the following errors:
FAILED: ninja -t msvc -e environment.x86 -- E:\b\build\goma/gomacc.exe "E:\b\depot_tools\win_toolchain\vs2013_files\VC\bin\amd64_x86/cl.exe" /nologo /showIncludes /FC @obj/third_party/webrtc/sound/rtc_sound/nullsoundsystem.obj.rsp /c ../../third_party/webrtc/sound/nullsoundsystem.cc /Foobj/third_party/webrtc/sound/rtc_sound/nullsoundsystem.obj /Fdobj/third_party/webrtc/sound/rtc_sound_cc.pdb
e:\b\build\slave\win_gn\build\src\third_party\webrtc\sound\nullsoundsystem.cc(78) : error C3861: 'LOG': identifier not found
FAILED: ninja -t msvc -e environment.x86 -- E:\b\build\goma/gomacc.exe "E:\b\depot_tools\win_toolchain\vs2013_files\VC\bin\amd64_x86/cl.exe" /nologo /showIncludes /FC @obj/third_party/webrtc/sound/rtc_sound/platformsoundsystemfactory.obj.rsp /c ../../third_party/webrtc/sound/platformsoundsystemfactory.cc /Foobj/third_party/webrtc/sound/rtc_sound/platformsoundsystemfactory.obj /Fdobj/third_party/webrtc/sound/rtc_sound_cc.pdb
e:\b\build\slave\win_gn\build\src\third_party\webrtc\sound\platformsoundsystemfactory.cc(29) : error C3861: 'LOG': identifier not found
ninja: build stopped: subcommand failed.

BUG=webrtc:4160
R=kjellander@webrtc.org
NOTRY=True

Review URL: https://codereview.webrtc.org/1419413002

Cr-Commit-Position: refs/heads/master@{#10408}
diff --git a/webrtc/sound/BUILD.gn b/webrtc/sound/BUILD.gn
index 82c6d77..43decf8 100644
--- a/webrtc/sound/BUILD.gn
+++ b/webrtc/sound/BUILD.gn
@@ -28,6 +28,9 @@
     "soundsystemproxy.h",
   ]
 
+  configs += [ "..:common_config" ]
+  public_configs = [ "..:common_inherited_config" ]
+
   if (is_clang) {
     # Suppress warnings from Chrome's Clang plugins.
     # See http://code.google.com/p/webrtc/issues/detail?id=163 for details.