Don't build sync_integration_tests by default for official builds.

I think this used to be built by the old Windows official builders, but
I have no idea if it was ever even used, and AFAICT, it hasn't ever been
built by the current (PGO) builders, and hasn't been missed. It seems we
don't really need it (especially not for the official continuous
builders, which appear to be the only ones that currently build the
"chrome_official_builder" target).

R=dimu@google.com, dpranke@google.com

Review-Url: https://codereview.chromium.org/2846963002
Cr-Commit-Position: refs/heads/master@{#467867}

BUILD.gn

@@ -758,7 +758,6 @@ if (is_win) {
       ":chrome_official_builder_no_unittests",
       "//base:base_unittests",
       "//chrome/test:browser_tests",
-      "//chrome/test:sync_integration_tests",
       "//ipc:ipc_tests",
       "//media:media_unittests",
       "//media/midi:midi_unittests",