Bug 1588160 - Increase test chunks for windows 10 asan browser-chrome; r=ahal

Avoid intermittent task timeouts by increasing test chunks.

Differential Revision: https://phabricator.services.mozilla.com/D49016

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

taskcluster/ci/test/mochitest.yml

@@ -103,6 +103,7 @@ mochitest-browser-chrome:
             linux64-asan/opt: 16
             macosx.*64/debug: 12
             windows10-64-ccov/debug: 14
+            windows10.*-asan/opt: 9
             default: 7
     max-run-time:
         by-test-platform: