Bug 1568309 - Increase chunks for linux64/asan wpt; r=jmaher

Increase test chunks to avoid intermittent task timeouts. At 18 chunks,
wpt-4 is the longest running, at about 90 minutes; 18 also aligns with
several /debug platforms.

Differential Revision: https://phabricator.services.mozilla.com/D39129

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/web-platform.yml

@@ -45,6 +45,7 @@ web-platform-tests:
     chunks:
         by-test-platform:
             android.*: 18
+            linux64-asan/opt: 18
             linux.*/debug: 18
             macosx*/opt: 8
             macosx.*/debug: 16