Use HpackEntry::Size() instead of rolling our own in QpackDynamicTable.

(Note that QpackEntry is an alias for HpackEntry.)

I'm a little uncomfortable with HpackEntry::Size() returning size_t instead of
uint64_t, because Chromium supports platforms where size_t is uint32_t and
sometimes funny things happen, especially with UBSAN around type conversions.
The reason for returning size_t is that is string_view::size_type and
std::string::size_type.  In practice we'll never deal with long strings so
this should really not be a problem.

This came up because I will need to access EntrySize() from outside
QpackHeaderTable and it makes more sense to use HpackEntry::Size() consistently
then to make EntrySize() public.

gfe-relnote: n/a, QUIC v99-only code.
PiperOrigin-RevId: 257903243
Change-Id: I354ffb46a9a6d7e2e4f2ba2d581115f6c0f4cfd5