[sync] Make SetSyncFeatureRequested() a no-op outside ChromeOS

No expected behavioral changes as
SyncServiceImpl::SyncAuthAccountStateChanged() has similar logic that
reacts to IdentityManager's ConsentLevel changing.

There are cases where SetSyncFeatureRequested() is invoked ahead of
ConsentLevel changes, e.g.
TurnSyncOnHelper::SigninAndShowSyncConfirmationUI(). However, it is
believed that those cases would be no-ops even before this patch because
SetInitialSyncFeatureSetupComplete() hasn't been invoked yet (outside
ChromeOS).

In upcoming patches, the function will be renamed and ifdef-ed for
ChromeOS only.

Change-Id: I673c6f2271a9c04f34da794239dbfdc34149a4df
Bug: 40772592
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6533710
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459298}