Increase angle_perftests shards on Android FYI 64 Perf (Pixel 2)

Currently we are hitting shard timeouts for angle_perftests on the
Android FYI 64 Perf (Pixel 2) bot. Even when we pass it's barely under the 20
minute shard time.

This adds a 4th shard for just that configuration.

TEST=angle_perftests

Bug: 1140626
Change-Id: Ib87d323a28e7c7703bc1af77246a89f2d8087413
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488044Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819152}

testing/buildbot/chromium.gpu.fyi.json

@@ -3014,7 +3014,7 @@
             }
           ],
           "service_account": "chromium-tester@chops-service-accounts.iam.gserviceaccount.com",
-          "shards": 3
+          "shards": 4
         },
         "test_id_prefix": "ninja://third_party/angle/src/tests:angle_perftests/"
       }

testing/buildbot/test_suite_exceptions.pyl

@@ -75,6 +75,13 @@
     },
   },
   'angle_perftests': {
+    'modifications': {
+     'Android FYI 64 Perf (Pixel 2)': { # crbug.com/1140626
+       'swarming': {
+         'shards': 4,
+       },
+     },
+    },
     'remove_from': [
       'Android FYI Release (Nexus 5)',  # crbug.com/915429
       'Android FYI Release (Nexus 6)',  # anglebug.com/2433