chromium.android: Bump shard count of net_unittests on tablet bots.

Bug: 796376
Change-Id: Ifb1b14bddfad4db4fdfcef966a760a31c6720b9e
Reviewed-on: https://chromium-review.googlesource.com/835647Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525248}

testing/buildbot/chromium.android.json

@@ -4945,7 +4945,7 @@
               "name": "shard #${SHARD_INDEX} logcats"
             }
           ],
-          "shards": 2
+          "shards": 3
         },
         "test": "net_unittests"
       },
@@ -6423,7 +6423,7 @@
               "name": "shard #${SHARD_INDEX} logcats"
             }
           ],
-          "shards": 2
+          "shards": 3
         },
         "test": "net_unittests"
       },
@@ -7857,7 +7857,7 @@
               "name": "shard #${SHARD_INDEX} logcats"
             }
           ],
-          "shards": 2
+          "shards": 3
         },
         "test": "net_unittests"
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -2754,19 +2754,16 @@
       'KitKat Tablet Tester': {
         'swarming': {
           'hard_timeout': 1800,
-          'shards': 2,
         },
       },
       'Lollipop Phone Tester': {
         'swarming': {
           'hard_timeout': 1800,
-          'shards': 2,
         },
       },
       'Lollipop Tablet Tester': {
         'swarming': {
           'hard_timeout': 1800,
-          'shards': 2,
         },
       },
       'Marshmallow 64 bit Tester': {