[ios] Splits Bookmarks EG tests into 5 test cases.

Also changes some BookmarksTestCase instance methods to class methods to be shared amongst test
cases.

Bug: 853348
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I99f57091ff0a05a9caa90a60074c0da77cf68920
Reviewed-on: https://chromium-review.googlesource.com/1103610
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568078}

ios/build/bots/tests/eg_tests.json

@@ -10,6 +10,7 @@
     },
     {
       "app": "ios_chrome_bookmarks_egtests",
+      "shards": 2,
       "xctest": true
     },
     {