Bug 1549619 - task efficiency: revisit chunking of mochitest-devtools-chrome r=jmaher

Changes:
- reduced `linux64/debug` chunking to 14 (from 16)
- reduced `linux64-ccov/debug` to 14 (from 16)
- increased `windows64-ccov` to 16 (from 10)
- maintain all non-ccov debug to 8
- maintain all asan to 8
- reduce everything else to 5

Differential Revision: https://phabricator.services.mozilla.com/D30303

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

taskcluster/ci/test/mochitest.yml

@@ -222,11 +222,11 @@ mochitest-devtools-chrome:
             default: 5400
     chunks:
         by-test-platform:
-            linux64/debug: 16
-            linux64-ccov/debug: 16
-            windows10-64-ccov/debug: 10
-            macosx64-ccov/debug: 15
-            default: 8
+            .*-ccov/debug: 16
+            linux64/debug: 12
+            macosx64/debug: 8
+            .*-asan/opt: 8
+            default: 5
     mozharness:
         mochitest-flavor: chrome
         chunked: true