The implementation of the StreamingDecoder depends on async compilation.
However, when the --single-threaded flag is set, async compilation is
not available. Therefore V8 does not support streaming compilation at
the moment if the --single-threaded flag is set.

This CL is the first step to support streaming compilation in
--single-threaded mode. This CL makes the StreamingDecoder an abstract
class, and the current implementation a sub-class called
AsyncStreamingDecoder. A follow-up CL will provided a second sub-class
implementation for streaming compilation in --single-threaded mode.

Bug: v8:10548
Change-Id: Ice5c01340d3df18f836a4a05d30571207ca8ccf6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208869
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67931}