Remove Clipboard::RawData from Clipboard::Data variant

This CL removes the `RawData` variant of `Clipboard::Data` and passes it
as a separate parameter to `WritePortableAndPlatformRepresentations`.
This is done to allow multiple `WriteData` calls to be made on the same
OS clipboard write, which is required by the fix to crbug.com/380236323.

See crrev.com/c/6077543 for extra context

Bug: 380236323
Change-Id: Id336de6cdc80ac44ba54ac3e693540fdcd27b2b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6164476
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1404992}