Disable linux_aura on the main builders.

Currently, browser_tests are running 2x-3x slower on the bots. Our first
dev channel has been very enlightening and got people to file all sorts
of helpful bugs.

BUG=319075,125106,303342,318961
NOTRY=True
R=cmp@chromium.org

Review URL: https://codereview.chromium.org/73323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235280 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235280