Revert 245139 "Add libwebm in third_party."

Causing 'check_license' failure on 'Linux' builder
http://build.chromium.org/p/chromium/builders/Linux/builds/46649

> Add libwebm in third_party.
> 
> libwebm will be used by chromoting client to generate WebM stream for
> MediaSource API.
> 
> BUG=321825
> 
> Review URL: https://codereview.chromium.org/134643003

TBR=sergeyu@chromium.org

Review URL: https://codereview.chromium.org/139993005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245187 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245188 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245188