Increase shared count of non_network_service_browser_tests

This CL increases the shard count from 10 to 21, which matches the
count specified in test_suite_exceptions.pyl.

TBR=jbudorick@chromium.org

Bug: 938656
Change-Id: I37bfa264ea7ee37fb092eec5f8b4267adb2ea2ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504978Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638036}

testing/buildbot/chromium.chromiumos.json

@@ -585,7 +585,7 @@
         "name": "non_network_service_browser_tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
-          "shards": 10
+          "shards": 21
         },
         "test": "browser_tests"
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -949,6 +949,16 @@
       },
     },
   },
+  'non_network_service_browser_tests': {
+    'modifications': {
+      # chromium.chromiumos
+      'linux-chromeos-dbg': {
+        'swarming': {
+          'shards': 21,
+        },
+      },
+    },
+  },
   'non_network_service_webkit_layout_tests' : {
     'remove_from': [
       'Linux Tests (dbg)(1)(32)', # 32-bit linux is unsupported