Increase WPT cq/waterfall shards from 10 to 15.

A couple of shards were bumping up to the 60m timeout, so this should
speed things up.

Change-Id: I622ca26eab04dad037ef2b94a80cc217e8360f03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2051143Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740658}

testing/buildbot/chromium.fyi.json

@@ -21915,7 +21915,7 @@
               "os": "Ubuntu-16.04"
             }
           ],
-          "shards": 10
+          "shards": 15
         },
         "test_target": "//:wpt_tests_isolate"
       }

testing/buildbot/test_suites.pyl

@@ -3545,7 +3545,7 @@
         },
         'isolate_name': 'wpt_tests_isolate',
         'swarming': {
-          'shards': 10,
+          'shards': 15,
         },
       },
     },