Making the fetch code work with BlobHandle (held as |blob| in ServiceWorkerResponse in fetch handling case) is critical to make Blob lifetime management work saner esp. when we can't easily rely on implicit IPC ordering assumptions with Browser process (e.g. in Servicification context).

Currently we pass ServiceWorkerResponse via traditional IPC if (and only if) the response has a valid blob uuid (and blob ptr is non-null if kMojoBlobs is enabled), but this code currently doesn't work if we try to pass the response via ServiceWorkerFetchResponseCallback.

Currently it hits serialization / deserialization error in NativeStructSerializer:

https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/lib/native_struct_serialization.h

It looks ParamTraits::GetSize and ::Write for BlobHandle result in different size (attachment may not be supported in native struct in mojo?):

https://cs.chromium.org/chromium/src/content/common/content_param_traits.cc?sq=package:chromium&dr&q=BlobHandle&l=115

ServiceWorkerResponse.blob and storage::BlobHandle are introduced by this change by mek@:
https://chromium-review.googlesource.com/c/567549