Bug 1550915 - task efficiency: re-balance reftest and reftest-no-accel chunking on linux r=gbrown

Changes:
- linux64 chunks reduced to 5 (from default 8)
- linux64 no-accel chunks reduced to 4 (from default 8)

All non-Linux chunks are untouched.

Differential Revision: https://phabricator.services.mozilla.com/D30749

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

taskcluster/ci/test/reftest.yml

@@ -117,6 +117,7 @@ reftest:
             android-em-4.3-arm7-api-16/debug: 56
             android-em-4.*: 28
             android-em-7.*: 5
+            linux64(-shippable|-devedition|-qr)?/opt: 5
             macosx64.*/opt: 2
             macosx64.*/debug: 3
             macosx64-ccov/debug: 6
@@ -177,6 +178,7 @@ reftest-no-accel:
         by-test-platform:
             macosx.*: 1
             windows.*: 4
+            linux64(-shippable|-devedition)?/opt: 4
             default: 8
     mozharness:
         chunked: