WebTransport: Hook up outgoing datagram attributes

Connect outgoingMaxAge and outgoingHighWaterMark attributes on
DatagramDuplexStream to WebTransport, so that setting them works as
expected.

To support the existing non-standard datagramWritableHighWaterMark
WebTransport option, its value is passed to DatagramDuplexStream on
construction. DatagramUnderlyingSink now reads the live value from the
DatagramDuplexStream object, so there are no synchronisation issues.

Changes to outgoingHighWaterMark are forwarded to the non-standard
WebTransport setDatagramWritableQueueExpirationDuration() method, which
will likely be renamed and removed from the IDL before shipping. This
results in a Mojo call to the network service where the expiration is
actually implemented.

In order to test the call to
setDatagramWritableQueueExpirationDuration() we now need a properly
initialised WebTransport object. To do this, factor out some code from
the BidirectionalStream test ScopedWebTransport class into a new
"TestWebTransportCreator" class in test_utils.h. Then reuse that code in
the DatagramDuplexStream tests.

Bug: 1011392
Change-Id: Ied395f157c80bc580dbb0fa30050666bb6e3a09a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2960704


Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#893364}