increase shard for pixel_test on android-marshmallow-arm64-rel

The test is slow on android.
https://chromium-swarm.appspot.com/tasklist?c=name&c=duration&c=state&d=desc&f=pool-tag%3AChrome&f=state%3ACOMPLETED_SUCCESS&f=name-tag%3Apixel_test&k=name-tag&s=duration

Bug: 941946
Change-Id: I142e57e41662d0ccfa8215c06e5ec6091c3c4cc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642754
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665956}

testing/buildbot/chromium.gpu.json

@@ -229,7 +229,8 @@
               "os": "Android"
             }
           ],
-          "idempotent": false
+          "idempotent": false,
+          "shards": 2
         }
       },
       {

testing/buildbot/test_suite_exceptions.pyl

@@ -1216,6 +1216,15 @@
       },
     },
   },
+ 'pixel_test':{
+    'modifications': {
+      'Android Release (Nexus 5X)': {
+        'swarming': {
+          'shards': 2,
+        },
+      },
+    },
+ },
  'remoting_unittests': {
     'remove_from': [
       # chromium.linux