[Infra] Shard content_browsertests on one ASAN trybot.

content_browsertests is slow on Linux Chromium OS ASan LSan Tests (1),
causing test timeout. This CL shards the test target.

Bug: 822461
Change-Id: I5924dc7deaae63b3a0709a0c97b4d1b46d1fbbe8
Reviewed-on: https://chromium-review.googlesource.com/973555
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544803}

testing/buildbot/chromium.memory.json

@@ -1642,7 +1642,8 @@
       },
       {
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "shards": 4
         },
         "test": "content_browsertests"
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -1165,6 +1165,12 @@
           'shards': 4,
         },
       },
+      'Linux Chromium OS ASan LSan Tests (1)': {
+        # content_browsertests is slow on ASAN try bot. crbug.com/822461.
+        'swarming': {
+          'shards': 4,
+        }
+      },
       'Mac ASan 64 Tests (1)': {
         'swarming': {
           'shards': 2,