Add an optional task slice for L phone dbg tester that runs on LMY48M.

This will help move its tests over to LMY48M from LMY48I, for
consistency with the the release tester.

Bug: 1091539
Change-Id: Icbf74b9c766e836f74cf61abafb92861a8e62871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252840Reviewed-by: Haiyang Pan <hypan@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779998}

testing/buildbot/waterfalls.pyl

@@ -392,7 +392,8 @@
         'swarming': {
           'dimension_sets': [
             {
-              'device_os': 'LMY48I',
+              # TODO(crbug.com/1091539): Move these over to LMY48M.
+              'device_os': 'LMY48M|LMY48I',
               'device_type': 'hammerhead',
               'os': 'Android',
             },