h5vcc: Use base::span for data transport in H5vccPlatformService

[rakuco]

Use a ReadOnlyBuffer mojom type to represent data rather than an array, as this makes the C++ bindings use base::span instead of std::vector.

Doing so allows for less duplication and fewer manual conversions, as DOMArrayBuffer can deal with spans natively, and some simplification on the browser side as well. While here, const-ify and use std::unique_ptr with base::FreeDeleter on response_ptr to avoid manual memory management.

Tested with the softmic extension on Linux.

Fixed: 487712437

[github-actions]

🤖 Gemini Suggested Commit Message

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cobalt: Use base::span for H5vcc data transport

Update the H5vccPlatformService Mojo interface to use
mojo_base.mojom.ReadOnlyBuffer for data transport instead of array<uint8>.
This ensures that the C++ bindings utilize base::span directly, reducing
duplication and manual conversions, as DOMArrayBuffer can deal with spans
natively. This also simplifies browser and renderer side code.

Additionally, this change introduces std::unique_ptr with base::FreeDeleter
for managing response memory on the browser side, improving safety.

Bug: 487712437

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[gemini-code-assist]

Code Review

This pull request refactors the H5vccPlatformService to use base::span for data transport over Mojom by switching from array<uint8> to mojo_base.mojom.ReadOnlyBuffer. This is a good improvement that reduces data copying. The change also introduces std::unique_ptr with base::FreeDeleter to manage memory returned from the Starboard service, which improves memory safety by replacing manual free() calls. The changes are well-implemented across the C++ and Mojom files. I have one suggestion to ensure type safety during a numeric conversion.

[rakuco]

/gemini review

[rakuco]

cc @abhijeetk @jkim-julie @Gyuyoung