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The remaining type-converter is for GpuMemoryBufferHandle. So introduce a
struct-trait for it instead. It also needs struct-traits for
NativePixmapPlane and NativePixmapHandle. Remove the type-converters,
since they are no longer needed. Also, expose NativePixmapPlane and
NativePixmapHandle types to non-ozone platforms, so we don't need some
special ozone-only mojom API.

BUG=637923
TBR=reveman@ for trivial components/exo change

Review-Url: https://codereview.chromium.org/2298003002
Cr-Commit-Position: refs/heads/master@{#416362}