Make sure the 'use_system_*' variables are actually visible in the relevant gyp files.

Retrying r34195, this time without libxslt, since we don't want to use the
system version of that if we're not also using system libxml (merged from
http://codereview.chromium.org/481011).

BUG=29738

Review URL: http://codereview.chromium.org/486012

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 003be53b23e2d6dff8cf65759ca68d0140224468