Mark sync_integration_tests as experimental on Win10 x64.

We seem to be seeing a situation where sync_integration_tests
is 4x to 5x slower and hitting errors on one set of our GCE
bots; in most cases, the test suite ends up timing out as a result.
The set of bots in question are being managed via a new system,
but apart from that, we don't know what's different about them yet.

However, since this test step is on a bot that's in the CQ, this
is causing a bunch of problems and increased cycle times. Short
of turning it off completely, we can at least mark the step as
experimental (which won't help w/ cycle times, but will at least
keep us from retrying the builds).

Better would be to shift these tests off of this set of bots, but
we're not sure if we can do that yet.

R=smut@google.com, trieb@chromium.org
BUG=933356

Change-Id: I326de2d614b65f331952009a9f5d6a37a6a6d1ed
Reviewed-on: https://chromium-review.googlesource.com/c/1480704
Reviewed-by: smut <smut@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634198}
2 files changed