Revert of Disable TwoClientSessionsSyncTest.SingleClientEnabledEncryptionAndChanged...

Revert of Disable TwoClientSessionsSyncTest.SingleClientEnabledEncryptionAndChanged on Linux. (patchset #3 id:40001 of https://codereview.chromium.org/634873002/)

Reason for revert:
I disabled the wrong test (same name). The right test to be disabled is handled in https://codereview.chromium.org/637513002/.

Original issue's description:
> Disable TwoClientSessionsSyncTest.SingleClientEnabledEncryptionAndChanged on Linux.
>
> In sync_integration_tests this test started failing at
> https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests/builds/14440
> and has been flaky ever since.
>
> NOTRY=true
> BUG=420979
> TBR=zea@chromium.org
>
> Committed: https://crrev.com/17d75d0075174e088c1b4e1009d772c209ef230d
> Cr-Commit-Position: refs/heads/master@{#298426}

TBR=zea@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=420979

Review URL: https://codereview.chromium.org/638563002

Cr-Commit-Position: refs/heads/master@{#298443}