Refactor ClipboardOzone::GetSource

The issue in b/345214239 was caused because ClipboardOzone's source
tracking logic is only partially implemented on non-LaCrOS and
doesn't have any code to clear its source when data is copied from
outside the browser on Linux.

The issue is fixed by tracking the source URL within the clipboard
itself instead of using an internal data member in ClipboardOzone for
non-LaCrOS Linux, similarly to how ClipboardWin and ClipboardMac work.
For LaCrOS, instead the existing logic that serializes a
`DataTransferEndpoint` is kept, but without any internal data members.

Bug: b/345214239
Change-Id: I2cb753a4092851f1903d31025658a5d660d1fe2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5598457


Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312730}