[WebSocket] Reduce data copy with mojo_base.mojom.ReadOnlyBuffer.

This patch changes OnDataFrame() |data| arg type from array<uint8> to
mojo_base.mojom.ReadOnlyBuffer so that we can avoid copy at
services/network/websocket.cc.

In order to use ReadOnlyBuffer move semantics at Blink, this patch
changes the mojo typemap and build rule, which includes making
ReadOnlyBuffer a part of mojo base dynamic library rather than
single static library (mojo/public/mojom/base/BUILD.gn).

third_party/blink/perf_tests/websocket/receive-arraybuffer-1MBx100.html:
ToT:        107 MB/s
This patch: 151 MB/s

Change-Id: If1e5ff5c9a7bdc2bf60a7cca4a787c5f6f75f1fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672535
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672436}