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These operations are modeled after the corresponding values from
NSDragOperation, but they don't have any corresponding meaning on the
Web and neither are they used in the browser.

Bug: 1093536
Change-Id: I8d8770d2b234c835444c198b2639597c20b8a512
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2562667Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#837013}