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(1) Simplify clipboard_constants code by consistently specifying
constants in clipboard_constants (as opposed to clipboard_format_type).

(2) Add comments to explain the need for separation between
clipboard_constants and clipboard_constants_mac.

No intended functional changes.

Change-Id: I8c9520341c4d90f1a071c65d22b91e91eca5d410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1886112Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710900}