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Update the comment and explain it a bit better. Bitmaps have been
supported in the Native Android clipboard since https://crrev.com/c/2147890.

While I'm unsure of the first instance of this comment, an early
iteration is active in March 2012(https://crrev.com/9264014),
and a recent, more well-documented change that expanded this comment
is available here(https://crrev.com/12633009)

Change-Id: I192d3abd050577c026f126ed99c9d9313756555c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382881
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802884}