Restructure GYP for vp9, opus and direct trace

This is needed to make the build more flexible for some use cases.

BUG=4185
R=andresp@webrtc.org, stefan@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34099004

Cr-Commit-Position: refs/heads/master@{#8290}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8290 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/codecs/opus/opus.gypi

@@ -19,7 +19,7 @@
            ]
         }, {
           'dependencies': [
-            '<(DEPTH)/third_party/opus/opus.gyp:opus'
+            '<(opus_dir)/opus.gyp:opus'
           ],
         }],
       ],

webrtc/modules/audio_coding/neteq/neteq.gypi

@@ -26,7 +26,7 @@
     ],
     'neteq_dependencies': [
       '<@(codecs)',
-      '<(DEPTH)/third_party/opus/opus.gyp:opus',
+      '<(opus_dir)/opus.gyp:opus',
       '<(webrtc_root)/common_audio/common_audio.gyp:common_audio',
       '<(webrtc_root)/system_wrappers/system_wrappers.gyp:system_wrappers',
       'audio_decoder_interface',
@@ -55,7 +55,7 @@
         ],
       },
       'export_dependent_settings': [
-        '<(DEPTH)/third_party/opus/opus.gyp:opus',
+        '<(opus_dir)/opus.gyp:opus',
       ],
       'sources': [
         'interface/neteq.h',