fix build when ffmpeg, libvpx and libwebm are disabled

build/gn_migration.gypi

Index: build/gn_migration.gypi
diff --git a/build/gn_migration.gypi b/build/gn_migration.gypi
index a9555ab704598d226a8339bff4de15dbf5a8152e..279d4c47f528e6e4a826ffe8a7a90ec8c0be3c3e 100644
--- a/build/gn_migration.gypi
+++ b/build/gn_migration.gypi
@@ -19,6 +19,9 @@
 # above contracts.
 
 {
+  'includes': [
+    '../media/media_variables.gypi'
+  ],
   'targets': [
     {
       'target_name': 'gyp_all',
@@ -360,7 +363,6 @@
             '../gpu/gles2_conform_support/gles2_conform_test.gyp:gles2_conform_test',  # TODO(GYP) crbug.com/471920
             '../gpu/khronos_glcts_support/khronos_glcts_test.gyp:khronos_glcts_test',  # TODO(GYP) crbug.com/471903 to make this complete.
             '../ipc/ipc.gyp:ipc_perftests',
-            '../media/media.gyp:ffmpeg_regression_tests',  # TODO(GYP) this should be conditional on media_use_ffmpeg
             '../mojo/mojo_base.gyp:mojo_common_unittests',
             '../ppapi/tools/ppapi_tools.gyp:pepper_hash_for_uma',
             '../skia/skia.gyp:filter_fuzz_stub',
@@ -379,6 +381,11 @@
             '../ui/views/examples/examples.gyp:views_examples_with_content_exe',
           ],
         }],
+        ['media_use_ffmpeg==1', {
+          'dependencies': [
+            '../media/media.gyp:ffmpeg_regression_tests',
+          ],
+        }],
         ['OS=="android" or OS=="linux"', {
           'dependencies': [
             '../net/net.gyp:disk_cache_memory_test',