Bug 1238948 - Switch TC Linux64 debug mochitests to 8 chunks. r=jmaher

--HG--
extra : rebase_source : 256983b5b3da748afe7fe23f6cb7132596dfdc18

testing/taskcluster/tasks/tests/fx_linux64_mochitest_plain.yml

@@ -15,7 +15,7 @@ task:
         loopbackAudio: true
   extra:
     chunks:
-      total: 5
+      total: 8
     suite:
       name: mochitest
       flavor: plain-chunked