Since Metal code frequently uses Objective-C types like id<MTLTexture>
etc. this migrates Metal translation units to Obj-C++. In particular,
this allows transitively including headers that include such types. This
in turn helps with refactoring `EmbedderTest`, `TestMetalContext`,
`TestMetalSurface` to avoid specifying `void*` types in headers and
manually refcounting via ARC bridge casts.
Reformats the source, since Objective-C files are linted to 100 columns
rather than the 80 column limit we impose on C++ files.
This change introduces no semantic changes.
Issue: https://github.com/flutter/flutter/issues/137801
c74f169f93