linux: only use hardcoded font list in layout test mode
linux: only use hardcoded font list in layout test mode

It's helpful to be able to use test_shell to debug problems seen
with a more complicated system fontconfig.  You can always pass
--layout-test-mode (which is what layout tests do) to get the old behavior
back.

I lost like two hours to this today.  :(
Even wrote a separate small program to use fontconfig to see why it
wasn't pulling out fonts I knew were available...

Review URL: http://codereview.chromium.org/521038

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@35601 0039d316-1c4b-4281-b951-d872f2087c98

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