Changes to media.gyp for Chromoting iOS client

This makes the following changes to media.gyp to make it compile for iOS (for
the Chromoting iOS client):

- Don't use ffmpeg or libvpx in media (just like Android)
- Do use libyuv
- Add yasm flag to decorate functions correctly for iOS (just like in the Mac)

BUG=134226

Review URL: https://codereview.chromium.org/474693002

Cr-Commit-Position: refs/heads/master@{#289973}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289973 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed