Bug 1347269 - Increase taskcluster instance size for linux twitch tes…

…ts. r=dustin

This attempts to address intermittent twitch test failures on Linux by having
the tests run on beefier hardware. Bug 1344961 shows a number of failures
where tests timeout before page loading is complete, the hope here is that
more powerful hardware speed up these load times.

MozReview-Commit-ID: 2yBQvKS8Xlh

--HG--
extra : rebase_source : 2b803204e90a736b4634279ca8199cb8910551ba

taskcluster/ci/test/tests.yml

@@ -122,6 +122,10 @@ external-media-tests-twitch:
     treeherder-symbol: tc-VP(b-t)
     unittest-try-name: media-twitch-tests
     e10s: false
+    instance-size:
+        by-test-platform:
+            linux.*: xlarge
+            default: default
     tier: 2
     max-run-time: 5400
     docker-image: {"in-tree": "desktop1604-test"}