Update libvpx.gyp for removed third_party libmkv.

libvpx.gyp

Index: libvpx.gyp
===================================================================
--- libvpx.gyp	(revision 286314)
+++ libvpx.gyp	(working copy)
@@ -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!': [