Update libvpx.gyp for removed third_party libmkv.

BUG=387738
TBR=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/427943002

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@286315 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
diff --git a/libvpx.gyp b/libvpx.gyp
index bb5d5f8..321af9b 100644
--- a/libvpx.gyp
+++ b/libvpx.gyp
@@ -181,14 +181,6 @@
                 '-Wno-parentheses-equality',
               ],
             }],
-            ['chromeos == 1', {
-              # ChromeOS needs these files for animated WebM avatars.
-              'sources': [
-                '<(libvpx_source)/third_party/libmkv/EbmlIDs.h',
-                '<(libvpx_source)/third_party/libmkv/EbmlWriter.c',
-                '<(libvpx_source)/third_party/libmkv/EbmlWriter.h',
-              ],
-            }],
           ],
         },
       ],
@@ -352,14 +344,6 @@
                 '../../build/android/cpufeatures.gypi',
               ],
             }],
-            ['chromeos == 1', {
-              # ChromeOS needs these files for animated WebM avatars.
-              'sources': [
-                '<(libvpx_source)/third_party/libmkv/EbmlIDs.h',
-                '<(libvpx_source)/third_party/libmkv/EbmlWriter.c',
-                '<(libvpx_source)/third_party/libmkv/EbmlWriter.h',
-              ],
-            }],
             ['OS == "ios"', {
               'xcode_settings': {
                 'OTHER_CFLAGS!': [