[ios][web] Put member fields initializer in declaration

Move the initialization of the member fields of NavigationManagerImpl
to the declaration (this is a cleanup to make it obvious all of them
are properly initialised all the time).

Bug: none
Change-Id: I750a51e34ced8a0f2a14cb821c77bdd4b4a37238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4517416
Reviewed-by: Federica Germinario <fedegermi@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1143985}