Remove use_system_ffmpeg-related logic from media and other mainline gyp files.

media/media.gyp

Index: media/media.gyp
diff --git a/media/media.gyp b/media/media.gyp
index 5f38e3efcc86553e68e0682a3d6ae72853f89f79..557d118c3735cf10e0455212f7fadfb0af779cca 100644
--- a/media/media.gyp
+++ b/media/media.gyp
@@ -510,28 +510,6 @@
             'filters/vpx_video_decoder.h',
           ],
         }],
-        ['use_system_ffmpeg == 1', {
-          'cflags': [
-            '<!(python <(DEPTH)/tools/compile_test/compile_test.py '
-                '--code "#define __STDC_CONSTANT_MACROS\n'
-                '#include <libavcodec/avcodec.h>\n'
-                'int test() { return AV_CODEC_ID_OPUS; }" '
-                '--on-failure -DCHROMIUM_OMIT_AV_CODEC_ID_OPUS=1)',
-
-            '<!(python <(DEPTH)/tools/compile_test/compile_test.py '
-                '--code "#define __STDC_CONSTANT_MACROS\n'
-                '#include <libavcodec/avcodec.h>\n'
-                'int test() { return AV_CODEC_ID_VP9; }" '
-                '--on-failure -DCHROMIUM_OMIT_AV_CODEC_ID_VP9=1)',
-
-            '<!(python <(DEPTH)/tools/compile_test/compile_test.py '
-                '--code "#define __STDC_CONSTANT_MACROS\n'
-                '#include <libavcodec/avcodec.h>\n'
-                'int test() { struct AVFrame frame;\n'
-                'return av_frame_get_channels(&frame); }" '
-                '--on-failure -DCHROMIUM_NO_AVFRAME_CHANNELS=1)',
-          ],
-        }],
         ['OS == "ios"', {
           'includes': [
             # For shared_memory_support_sources variable.