Add shards to speed up ui_eg2tests

Bug: 1015953
Change-Id: Iee4ac7fb72aaabefdedd59b14a30503aac17779b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892454
Auto-Submit: Lindsay Pasricha <lindsayw@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Lindsay Pasricha <lindsayw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713048}

ios/build/bots/tests/eg2_tests.json

@@ -17,7 +17,8 @@
         },
         {
             "app": "ios_chrome_ui_eg2tests_module",
-            "host": "ios_chrome_eg2tests"
+            "host": "ios_chrome_eg2tests",
+            "shards": 2
         },
         {
             "app": "ios_chrome_web_eg2tests_module",