Shard net_unittests on ToTLinuxASan 16 ways to match memory bot

BUG=chromium:794372
R=thakis@chromium.org
NOTRY=True

Change-Id: Iaef5d1b0e4fcb155d90de714187e16496fdfdee1
Reviewed-on: https://chromium-review.googlesource.com/1015847
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551732}

testing/buildbot/chromium.clang.json

@@ -6839,7 +6839,7 @@
       {
         "swarming": {
           "can_use_on_swarming_builders": true,
-          "shards": 4
+          "shards": 16
         },
         "test": "net_unittests"
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -2307,8 +2307,9 @@
       },
       # chromium.clang
       'ToTLinuxASan': {
+        # TODO(crbug.com/794372): net_unittests is slow under ASan.
         'swarming': {
-          'shards': 4,
+          'shards': 16,
         },
       },
       # chromium.memory