Cherry-pick of 5df998d086 onto 2.4.x branch.
DEPRECATION:
Use `RouterModule.forRoot(routes, {initialNavigation: 'enabled'})` instead of
`RouterModule.forRoot(routes, {initialNavigtaion: true})`.
Before doing this, move the initialization logic affecting the router
from the bootstrapped component to the boostrapped module.
Similarly, use `RouterModule.forRoot(routes, {initialNavigation: 'disabled'})`
instead of `RouterModule.forRoot(routes, {initialNavigation: false})`.
Deprecated options: 'legacy_enabled', `true` (same as 'legacy_enabled'),
'legacy_disabled', `false` (same as 'legacy_disabled').
The "Router Initial Navigation" design document covers this change.
Read more here:
https://docs.google.com/document/d/1Hlw1fPaVs-PCj5KPeJRKhrQGAvFOxdvTlwAcnZosu5A/edit?usp=sharing
