Bug 1361476 - Change instance type for linux64-asan mochitest-devtools tests; r=ryanvm

For the last month or so, asan mochitest-devtools tests have been running on
xlarge instances, in an experiment. Since there is no conclusive evidence that
xlarge improved results, I am putting this back the way I found it.

taskcluster/ci/test/tests.yml

@@ -767,8 +767,8 @@ mochitest-devtools-chrome:
                     - --mochitest-suite=mochitest-devtools-chrome-chunked
     instance-size:
         by-test-platform:
-            # Bug 1361476 - try xlarge on asan to see if it avoids OOM
-            linux64-asan/opt: xlarge
+            # Bug 1281241: migrating to m3.large instances
+            linux64-asan/opt: legacy
             default: default
     # Bug 1296086: high number of intermittents observed with software GL and large instances
     allow-software-gl-layers: false