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

This will be removed in the future once errors due to isolate corruption
disappear.

R=bpastene@chromium.org

Bug: 1013093
Change-Id: I24976ec89c2a34900cade8e549766bb10858b45e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856871Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Anushruth  <anushruth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706018}

testing/buildbot/chromium.linux.json

@@ -2649,6 +2649,7 @@
               "os": "Ubuntu-16.04"
             }
           ],
+          "idempotent": false,
           "shards": 6
         },
         "test": "content_browsertests"

testing/buildbot/test_suite_exceptions.pyl

@@ -442,6 +442,11 @@
           'shards': 2, # https://crbug.com/955853
         },
       },
+      'Linux Tests': {
+        'swarming': {
+          'idempotent': False, # https://crbug.com/1013093
+        },
+      },
     },
   },
   'content_shell_crash_test': {