f01b0705eb
Extracts creation of graphics backend specfic EmbedderTestContext creation to their own translation units. In particular, this allows for less conditional header includes, and more specifically, for code relating to the Metal backend to include headers that include Objective-C types -- today we cast these all to void* to avoid declaring them in headers, which requires special handling for ARC. An alternative approach would have been to extract backend-specific subclasses, but there are test suites such as `EmbedderTestMultiBackend` that are executed against multiple backends, which make that approach impractical. No test changes since this patch makes no semantic changes. Issue: https://github.com/flutter/flutter/issues/137801 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style