http mojom: represent request header values as ByteStrings

This change passes the value field of HttpRequestHeaderKeyValuePair as a
ByteString rather than a string. The relevant standard (RFC 7230)
defines this as basically an opaque octet string with no defined
encoding; Blink internally treats them as being either Latin-1 or UTF-16
depending on whether they came over the wire or from JavaScript.

This change also adds some documentation of the WTF::String <-> mojo
string mapping.

Change-Id: I709ca1287b4d96628c74546ac5e99019522899fd
Bug: 326376204
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5372964
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1273039}