Change the content_shell_fonts hook to only run on linux.

This changes the gclient hook to run only conditionally on
linux, which keeps it from running and printing stuff on
the other platforms.

R=jbudorick@chromium.org

Change-Id: Id1dbfb50f139308b3c39875e00294ea9a0a76016
Reviewed-on: https://chromium-review.googlesource.com/831219Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524608}

DEPS

@@ -947,9 +947,9 @@ hooks = [
   {
     'name': 'content_shell_fonts',
     'pattern': '.',
+    'condition': 'checkout_linux',
     'action': [ 'download_from_google_storage',
                 '--no_resume',
-                '--platform=linux*',
                 '--extract',
                 '--no_auth',
                 '--bucket', 'chromium-fonts',