Initialize Sanitizer API implementation with partially implemented
constructor and `get()` methods of the `Sanitizer` interface, which
allows the sub-sequential implementation to be tested by WPT.
The `Sanitizer` interface is hidden behind the feature flag
`dom_sanitizer_enabled`, which is disabled by default.
Specification: https://wicg.github.io/sanitizer-api/
Testing: Enable WPT tests for Sanitizer API.
Fixes: Part of #43948
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Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>