Disable Drive integration for incognito.

https://crrev.com/c/1926146 introduced a bug that
util::IsDriveEnabledForProfile() might return true for incognito
profiles that are associated with Gaia user that leads to crashes.
Previously such check existed in IsProfileAssociatedWithGaiaAccount()
but was mistakenly treated as redundant.

Bug: 1034621
Change-Id: I041b418f16204db4680645e7cd8a3321da3ebc2d
Tests: Manual.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003852
Auto-Submit: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732408}

chrome/browser/chromeos/drive/file_system_util.cc

@@ -85,6 +85,9 @@ bool IsDriveEnabledForProfile(Profile* profile) {
   }
   if (!chromeos::LoginState::IsInitialized())
     return false;
+  // Disable Drive for incognito profiles.
+  if (profile->IsOffTheRecord())
+    return false;
   const user_manager::User* user =
       chromeos::ProfileHelper::Get()->GetUserByProfile(profile);
   if (!user || !user->HasGaiaAccount())