Store Memory Blob for script thread with more File information

[CYBAI]

As I mentioned in #22736 (comment), after implementing FormData in Vec<(LocalName, FormDatum)> in #22736, we only passed the keys test case. We cannot pass these test cases which has values comparison with File because we don't have the same lastModified and filename between BlobImpl::Memory and the File object constructed by File constructor.

In Gecko and Blink, both of them have a ToFile function for Blob

• https://searchfox.org/mozilla-central/source/dom/base/FormData.cpp#28
• https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/html/forms/form_data.cc?l=359

In Servo, we have a similar thing which is the BlobImpl::Memory; however, we only save the bytes for script thread.

servo/components/script/dom/blob.rs

Lines 39 to 40 in c3608c0

	/// Memory-based blob, whose content lives in the script process
	Memory(Vec<u8>),

So, to have more information for Blob in script thread, it's better to save more necessary fields like what FileBlob did.

We should have a new struct like MemoryBlob to save filename and last_modified timestamp (and more fields if needed). With this implementation, we should be able to pass the values related tests in xhr/formdata-foreach.html

Ex.

#[derive(JSTraceable)]
pub struct MemoryBlob {
    name: Option<DOMString>,
    cache: Vec<u8>,
    last_modified: Option<i64>,
}

#[derive(JSTraceable)]
pub enum BlobImpl {
    ...
    Memory(MemoryBlob),
    ...
}

impl BlobImpl {
    /// Construct memory-backed BlobImpl
    #[allow(unrooted_must_root)]
    pub fn new_from_bytes(name: DOMString, bytes: Vec<u8>, last_modified: i64) -> BlobImpl {
        BlobImpl::Memory(MemoryBlob {
            name,
            cache: bytes,
            last_modified
        })
    }
}