Add web shell test suite to the CQ and main waterfall.

This test has been stable on FYI, and having it on the CQ will
help catch regressions sooner.

Bug: 638141
Change-Id: Ie318f62993402a0f1b89f4aff47c821a0310722f
Reviewed-on: https://chromium-review.googlesource.com/557861Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Mike Baxley <baxley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485531}

ios/build/bots/tests/eg_tests.json

@@ -3,6 +3,10 @@
     {
       "app": "ios_chrome_integration_egtests",
       "xctest": true
+    },
+    {
+      "app": "ios_web_shell_egtests",
+      "xctest": true
     }
   ]
 }