Bug 1548160 - task efficiency: review and reduce chunk count of xpcshell for various platforms r=gbrown,jmaher

Changes:
- set `macosx64-ccov` tests to 8 chunks (unchanged from m-c)
- set windows and linux `ccov` to 6 chunks (from 8)
- set `linux64.*/debug` to 6 chunks (from 10 and 8)
- set `android-em` to 8 chunks (unchanged)
- set default chunks to be 5 (from 8)

Differential Revision: https://phabricator.services.mozilla.com/D29713

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

taskcluster/ci/test/xpcshell.yml

@@ -41,16 +41,15 @@ xpcshell:
             default: built-projects
     chunks:
         by-test-platform:
-            linux32/debug: 12
-            linux64/debug: 10
-            android-em-4.3-arm7-api-16/debug: 12
+            android-em-4.3-arm7-api-16/.*: 8
             android-em-7.*: 3
-            macosx.*: 1
-            windows.*: 1
-            windows10-aarch64/opt: 3
-            windows10-64-ccov/debug: 8
             macosx64-ccov/debug: 8
-            default: 8
+            macosx.*/.*: 2
+            linux64(-qr)?/debug: 6
+            (linux.*|windows.*)-ccov/debug: 6
+            windows(7-32|10-64)(-shippable|-devedition|-asan|.*-qr)?/.*: 2
+            windows10-aarch64/opt: 3
+            default: 5
     instance-size:
         by-test-platform:
             android-em.*: xlarge