Bug 1715306 - Rename Valgrind job to reflect running with Software WebRender. r=jmaher

Bug 1715132 ended up switching the Valgrind jobs to run with Software
WebRender. Let's explicitly mark the task as running with WebRender and
set the prefs to make this clear.

Differential Revision: https://phabricator.services.mozilla.com/D117188

taskcluster/ci/valgrind/kind.yml

@@ -18,14 +18,14 @@ job-defaults:
         retrigger: true
 
 jobs:
-    linux64-valgrind/opt:
-        description: "Linux64 Valgrind Opt"
+    linux64-valgrind-qr/opt-swr:
+        description: "Linux64 Valgrind Opt WebRender"
         index:
             product: firefox
-            job-name: linux64-valgrind-opt
+            job-name: linux64-valgrind-qr/opt-swr
         treeherder:
-            platform: linux64/opt
-            symbol: V
+            platform: linux64-qr/opt
+            symbol: V-swr
             kind: build
             tier: 1
         worker-type: b-linux-aws
@@ -41,6 +41,8 @@ jobs:
             config:
                 - builds/releng_base_firefox.py
                 - builds/releng_base_linux_64_builds.py
+            extra-config:
+                extra_prefs: [gfx.webrender.software=true]
             script: "mozharness/scripts/fx_desktop_build.py"
             secrets: true
             tooltool-downloads: public