Re-enable TwoClientDictionarySyncTest.Sanity_E2ETest test

Tested the test can pass on bot.

Bug: 1068415
Change-Id: I3a94ededc3969f3cdef01865ced46c2ad4d0e300
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2157783Reviewed-by: Ian Struiksma <ianstruiksma@google.com>
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761327}

testing/buildbot/filters/e2e.sync_integration_tests.filter

 # This is the filter file for sync e2e tests.
 
-# TODO(crbug.com/1068415): Disabled because it's flaky.
--TwoClientDictionarySyncTest.Sanity_E2ETest