Fix unsafe buffer usage in the Blink WebSocket module

- Removed `UNSAFE_TODO` marks.
- Spanified `WebSocketChannelImpl::ConsumeDataFrame`.
- Changed the `SegmentPtr` type to `base::HeapArray`.
- Changed `base::span<const char>` to `base::span<const uint8_t>`.

Bug: 351564777
Change-Id: Ic18ca57d6e0dd5253e04b13c3126a285ecf47956
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6917119
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: kelsen liu <kelsenliu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1513413}