mash/browser-tests: Use the correct test-flag for browser-test.
mash/browser-tests: Use the correct test-flag for browser-test.

'--use-x11-test-config' flag was replaced by '--use-test-config' in
https://crrev.com/388329, but it missed updating the flag used when running
browser-tests in mash.

BUG=594600

Review URL: https://codereview.chromium.org/1914283003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390060}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk