[code coverage] Fix sharding for coverage bots

This CL fixes an error in test spec for coverage bots so that test
targets can be correctly sharded.

Change-Id: I399d32917545a98951dc8ba0bb5ff51b84ccbc61
Reviewed-on: https://chromium-review.googlesource.com/c/1303043Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603285}

testing/buildbot/chromium.fyi.json

@@ -5856,9 +5856,9 @@
     "gtest_tests": [
       {
         "isolate_coverage_data": true,
-        "shards": 2,
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "shards": 2
         },
         "test": "base_unittests"
       }

testing/buildbot/test_suites.pyl

@@ -672,7 +672,9 @@
 
     'chromium_linux_coverage_gtests': {
       'base_unittests': {
-        'shards': 2,
+        'swarming': {
+          'shards': 2,
+        },
       },
     },