Cherry pick PR #8570: Allow render to fetch splash video on h5vcc schema and move hard coded webm to the embedded file

[linxinan-chops]

See original #8570:

Register the H5VCC URL schema to the fetch API at render side.

With the change, native splash implementation could call H5VCC to get the cached contents via fetch API.

In the Cherry pick, I have to refactor a new target for "//cobalt/shell:url_constants", as in M114 cobalt_shell_lib is still under "testonly = true", which blocks us to use the constants in production targets.

Issue: 454630524

[github-actions]

🤖 Gemini Suggested Commit Message

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cobalt: Register H5VCC scheme for Fetch API

Register the H5VCC URL scheme to enable the Fetch API within
the renderer. This allows native splash screen implementations to
fetch cached content via the H5VCC scheme.

The change required refactoring the url_constants into a new build
target, as cobalt_shell_lib is testonly in M114, preventing its use
in production targets.

Bug: 454630524

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[gemini-code-assist]

Code Review

This pull request is a cherry-pick to allow the renderer to fetch splash videos using the h5vcc schema. The changes correctly implement this by registering the h5vcc URL scheme with the Fetch API in the renderer process. The necessary refactoring of the BUILD.gn files to create a new url_constants target is also done correctly, allowing the renderer to access constants that were previously in a testonly target. The changes look good and are a correct application of the original patch to this branch.