[code health] s/ raw_ptr / unique_ptr / in BidirectionalStreamAdapter.

The high-level goal/benefit of this CL is removal of the
`AcrossTasksDanglingUntriaged` annotation from the
`BidirectionalStreamAdapter::bidirectional_stream_` field.

Before and after this CL the following holds:

* `<anonymous namespace>::BidirectionalStreamAdapter` from
  `components/grpc_support/bidirectional_stream_c.cc` has the same
  lifetime as `grpc_support::BidirectionalStream` from
  `components/grpc_support/bidirectional_stream.h`.  In particular,
  those 2 objects are always destroyed together - their destruction is
  initiated in `BidirectionalStreamAdapter::DestroyOnNetworkThread`.  In
  other words, `BidirectionalStreamAdapter` is effectively an owner of
  `BidirectionalStream`.
* `BidirectionalStream::delegate_` field is a non-owning `raw_ptr` that
  points to `BidirectionalStreamAdapter` (which implements/subclasses
  the `BidirectionalStream::Delegate`).

Before this CL `BidirectionalStreamAdapter::bidirectional_stream_` field
was a `raw_ptr<BidirectionalStream>`.  This meant that there existed a
cycle of `raw_ptr`s that point to each other - which created a situation
where neither can be deleted without creating a dangling pointer (see
https://chromium.googlesource.com/chromium/src/+/main/docs/dangling_ptr_guide.md#cyclic-pointers).

After this CL the type of the
`BidirectionalStreamAdapter::bidirectional_stream_` field is changed
from `raw_ptr<...>` to `std::unique_ptr<...>`.  This breaks the cycle of
`raw_ptr`s, which prevents the dangling pointer detector from firing.
Furthermore, this more accurately represents lifetime/object ownership
and avoids calling a raw `delete` on `BidirectionalStream` from
`BidirectionalStream::DestroyOnNetworkThread`.

Bug: 41485225
Change-Id: I7a6cf01aecbe9cb8022d3d100fcd2421d4349966
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6480655
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1450253}