Add APIs to ClipboardNonBacked to get/write ClipboardData.

This is being done to provide ash::ClipboardHistory the ability to
access and restore ClipboardData directly from ClipboardNonBacked
without having to otherwise read/write the clipboard buffer. Manually
reading/writing the buffer would likely be prone to missing some aspects
of ClipboardData if changes were to get out of sync.

Bug: 1104352
Change-Id: I80c800f1ebde26ad3bb8196bc0f438394bed2067
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316802
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792895}