Unify, simplify logic, reduce UTF8 specific handling.

Intend of this is also to have stream views.
Stream views can be used concurrently by multiple threads, but
only one thread may fetch new data from the underlying source.
This together with unified stream view creation is intended to be
used for parse tasks.

BUG=v8:6093

Change-Id: I3bce48185fa2c986d16619a9a8ece3ff4c4f5e60
Reviewed-on: https://chromium-review.googlesource.com/509489Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Cr-Commit-Position: refs/heads/master@{#45688}