SkStream is no longer a ref counted object.

With https://codereview.chromium.org/849103004/, SkStream is no longer
ref counted. Change callers that currently unref() SkStreams to one of
either:
- delete the stream
- pass ownership of the stream

screencap.cpp:
Call EncodeData directly, bypassing SkDynamicMemoryWStream and SkBitmap.

Utils.cpp:
Write directly to an SkData, and then use that to construct a new
SkStream.

Cherry-pick of a change that originally landed in master-skia and is
dependent on a skia merge (ag/655422).

Change-Id: Idc99ad7d5a70c893dc012d59915216f301ab3c9d