Because a StreamState object contains a unique_ptr, it is not
copyable. A vector of StreamStates, therefore, causes a compile error
on resize or push_back, both of which invoke the copy constructor.
I don't know why MSVS complains, but clang does not.
To fix this, I'm changing vector<StreamState> into deque<StreamState>.
At this point static_library builds are working in MSVS 2019.
shared_library builds are still not working.
Issue #867 (MSVS 2019)
Issue #336 (progress toward replacing Travis & Appveyor with GitHub
Actions, which uses MSVS 2019)
b/190743862 (internal; tracking replacement of Travis)
Change-Id: Iaa9d5fc357102d15eac96c29ebeee7c7236e976b
ae89145015