Add full definition of SkStreamAsset.

When moving from pointer to smart pointer, the implementation of
the pointee must be available (so the destructor which isn't used
can be visible).

Change-Id: I5489365868d4a3ba48cb4e8cd10e38b262dec2da
Reviewed-on: https://skia-review.googlesource.com/c/195129
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>