Make install-build-deps.sh install xfonts-mathml.

This provides the Symbol font, needed for tests.

BUG=316955

Review URL: https://codereview.chromium.org/72713004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235226 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235226