Add valgrind config into tools/mb/mb_config.pyl
Add valgrind config into tools/mb/mb_config.pyl

This CL adds valgrind config in mb_config.pyl for valgrind bots
on chromium.memory.fyi

R=dpranke@chromium.org
BUG=572694

Review URL: https://codereview.chromium.org/1648803002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372226}

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