chromeos: adds content_browsertests --mus to main waterfall (again)

Apparently the main waterfall now runs linux-chromeos-rel, so add
content_browsertests --mus to it.

BUG=776513
TEST=test only changes

Change-Id: I342b2a9ff107c870c15b9b8ffebd413ccc9d38f1
Reviewed-on: https://chromium-review.googlesource.com/773286Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517105}

testing/buildbot/chromium.chromiumos.json

@@ -1496,6 +1496,19 @@
         },
         "test": "content_browsertests"
       },
+      {
+        "args": [
+          "--mus",
+          "--test-launcher-filter-file=../../testing/buildbot/filters/mus.content_browsertests.filter"
+        ],
+        "name": "content_browsertests --mus",
+        "override_isolate_target": "content_browsertests",
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 2
+        },
+        "test": "content_browsertests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true,