Store inner whitespace of the body and compare it on conversion

Calculate and store the two inner whitespace values of the body in the
dm.Document. When converting back, make sure the first/last nodes
pre/post outer whitespace matches the inner left/right whitespace
of the body.

Bug: 54964
Change-Id: I45f1ffd63669f25a6cae878400bfe21719ed58ee
Merge "Store inner whitespace of the body and compare it on conversion"