Revert: Make content_browsertests on linux-rel and Linux Tests not idempotent.

This is revert of crrev.com/c/1856871. The issue with
content_browsertests seems to be resolved now.

R=bpastene@google.com

Bug: 1013093
Change-Id: I230fda96bbe6d42c7ddb0e4d739c737cb474ffdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947562Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Anushruth  <anushruth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721864}

testing/buildbot/chromium.linux.json

@@ -2598,7 +2598,6 @@
               "os": "Ubuntu-16.04"
             }
           ],
-          "idempotent": false,
           "shards": 6
         },
         "test": "content_browsertests"

testing/buildbot/test_suite_exceptions.pyl

@@ -517,11 +517,6 @@
           'shards': 2, # https://crbug.com/955853
         },
       },
-      'Linux Tests': {
-        'swarming': {
-          'idempotent': False, # https://crbug.com/1013093
-        },
-      },
       'Linux TSan Tests': {
         'swarming': {
           'shards': 15,