Add demo mode entry for IPH_PasswordsAccountStorage in about:flags

This was meant to be done in crrev.com/c/2220048, but a part of it was
overlooked there (that CL added only a DEFINE_VARIATION_PARAM(..) but
failed to also add the corresponding VARIATION_ENTRY(..) in the list).

Bug: 1044038
Change-Id: I54d11b14073f4908c634ba8539b870a0f5c6ec8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346433
Auto-Submit: Marc Treib <treib@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796882}

components/feature_engagement/public/feature_list.h

@@ -191,6 +191,7 @@ constexpr flags_ui::FeatureEntry::FeatureVariation
         VARIATION_ENTRY(kIPHDesktopTabGroupsNewGroupFeature),
         VARIATION_ENTRY(kIPHFocusModeFeature),
         VARIATION_ENTRY(kIPHGlobalMediaControls),
+        VARIATION_ENTRY(kIPHPasswordsAccountStorageFeature),
         VARIATION_ENTRY(kIPHReopenTabFeature),
         VARIATION_ENTRY(kIPHWebUITabStripFeature),
 #endif  // defined(OS_WIN) || defined(OS_APPLE) || defined(OS_LINUX) ||