QUIC - refactor stream creation. Guarded by --quic_reloadable_flag_quic_refactor_stream_creation.

QUIC - refactor stream creation. Guarded by --quic_reloadable_flag_quic_refactor_stream_creation.

This CL is in preparation for HTTP stream pairs changes.  A prototype
of the entire HTTP stream pairs work is in progress.  This CL is one
subset of that (2nd in the series).

[ HTTP stream pairs builds upon this, by having new QuicSpdySession
  overrides that implicitly create headers streams, and abstracting
  stream accounting as needed. ]

Add a new factory like method QuicSession::CreateStream(), that
unconditionally creates a new stream.  It's virtual so subclasses of
QuicSession can create streams that are appropriately specialized from
QuicStream.  There are new versions of Create*DynamicStream() called
MaybeCreate*DynamicStream(), and ShouldCreate*DynamicStream() called
ShouldCreate*DynamicStream2().

Separating this out means that fewer overrides of Create*DynamicStream()
and ShouldCreate*DynamicStream() will necessary, because many of those
were actually only needed to specialize the steam types.  Those will go
away when FLAGS_quic_reloadable_flag_quic_refactor_stream_creation is
deprecated.

Merge internal changes: 154351257, 154373998.

R=rch@chromium.org
BUG=

[Buck, 2017-05-03 19:03:05]

The CQ bit was checked by ckrasic@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-03 19:04:16]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-03 19:09:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-05-03 19:09:05]

Dry run: Try jobs failed on following builders:
  android_arm64_dbg_recipe on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_arm6...)
  linux_android_rel_ng on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/linux_androi...)
  chromeos_amd64-generic_chromium_compile_only_ng on
master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromeos_amd64-...)
  chromium_presubmit on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)
  linux_chromium_chromeos_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Ryan Hamilton, 2017-05-03 19:17:03]

lgtm

[Buck, 2017-05-03 21:46:59]

The CQ bit was checked by ckrasic@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-03 21:47:35]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-03 21:52:53]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-05-03 21:52:56]

Dry run: Try jobs failed on following builders:
  android_cronet on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_cron...)
  cast_shell_android on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/cast_shell_a...)

[Buck, 2017-05-03 22:03:31]

The CQ bit was checked by ckrasic@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-03 22:04:23]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-03 23:22:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-05-03 23:22:05]

Dry run: This issue passed the CQ dry run.