crostini: Wait for upgrade to finish instead of succeeding immediately.

Bug: chromium:930901
Test: Upgraded manually. Unit tests.
Change-Id: I227d354fc36d8527cadaf0a3c2465940703ee2f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1966800
Commit-Queue: David Munro <davidmunro@google.com>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724525}

chrome/browser/chromeos/crostini/crostini_upgrader.cc

@@ -188,9 +188,6 @@ void CrostiniUpgrader::OnUpgrade(CrostiniResult result) {
     }
     return;
   }
-  for (auto& observer : upgrader_observers_) {
-    observer.OnUpgradeSucceeded();
-  }
 }
 
 void CrostiniUpgrader::OnRestore(CrostiniResult result) {