Bug 1643177 - reduce web-platform-tests-backlog chunk counts r=jmaher

Changes:
  - reduce the number of web-platform-tests-backlog chunks across the board to save on machine setup costs.

Differential Revision: https://phabricator.services.mozilla.com/D79414

taskcluster/ci/test/web-platform.yml

@@ -87,16 +87,16 @@ web-platform-tests-backlog:
     treeherder-symbol: W-b(wpt)
     chunks:
         by-test-platform:
-            android.*: 6
+            android.*: 3
-            linux.*64-asan/opt: 6
+            linux.*64-asan/opt: 3
-            linux.*64-ccov/opt: 6
+            linux.*64-ccov/opt: 3
-            linux.*/debug: 6
+            linux.*/debug: 3
-            macosx.*/debug: 5
+            macosx.*/debug: 3
-            windows7.*/opt: 3
+            windows7.*/opt: 1
-            windows7.*/debug: 5
+            windows7.*/debug: 3
-            windows10.*/debug: 3
+            windows10.*/debug: 1
-            windows10-64-ccov/opt: 6
+            windows10-64-ccov/opt: 3
-            default: 3
+            default: 1
     max-run-time:
         by-test-platform:
             .*/debug: 7200