Don't attempt to store owner settings when IsOwner() == false

The existing code had a user_id_ check, but that is not sufficient for
owner settings writes to go through, the private key must be present
as well. Replace the check with a proper IsOwner() check. This
misbehavior is only observed in DeviceSettingsServiceTest.Observer,
getting exposed by a DCHECK added in [1]. It's hopefully not hit in
production (otherwise tentative owner settings might get lost), but
better to fix the root cause than avoiding the offending edge case in
the test.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2287290/11/chrome/browser/chromeos/ownership/owner_settings_service_chromeos.cc#800

BUG=chromium:1073940
TEST=unit tests pass

Change-Id: I6d5a5f17e8aeaa28600d57c7f0fcfad661434e33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611720Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840801}

chrome/browser/chromeos/ownership/owner_settings_service_chromeos.cc

@@ -739,7 +739,7 @@ void OwnerSettingsServiceChromeOS::ReloadKeypairImpl(
 
 void OwnerSettingsServiceChromeOS::StorePendingChanges() {
   if (!HasPendingChanges() || store_settings_factory_.HasWeakPtrs() ||
-      !device_settings_service_ || user_id_.empty()) {
+      !device_settings_service_ || user_id_.empty() || !IsOwner()) {
     return;
   }