Bug 1582970 - Increase max-run-time for linux64 ccov browser-chrome; r=marco

The recent increase was not quite enough: Allow another 30 minutes for linux64
ccov browser-chrome.

Differential Revision: https://phabricator.services.mozilla.com/D46787

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

taskcluster/ci/test/mochitest.yml

@@ -109,7 +109,7 @@ mochitest-browser-chrome:
             default: 7
     max-run-time:
         by-test-platform:
-            linux64-ccov/debug: 7200
+            linux64-ccov/debug: 9000
             windows7-32/debug: 5400
             windows10-64/debug: 5400
             windows10-64-ccov/debug: 10800