Remove use of "generic" MojoUnion::New() in PageState serialization.

These methods have a generic-sounding name but actually create a union
containing the first declared subtype. In general, this behavior is
non-intuitive; in many cases, this behavior is also inefficient, since
it allocates storage which may then be immediately deallocated.

Bug: 1261169
Change-Id: I6b8a34490b990d1fa1924f1b2795b364e38dea6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3597646
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#994591}