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Under chrome/browser/resources/pdf directory, move existing enums into
constants.js. constants.js will be used to hold all future enums which
are added into this directory.

Change-Id: Ic3ed8e0d0cfa13e157c45ea640ec52dd5a67b90e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056568Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Hui Yingst <nigi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743102}