Asking bots how they'd feel about it in https://codereview.chromium.org/2579693005/

I've taken a look at '.MOUSEDOWN' matches in httparchive:har.2016_11_15_chrome_requests_bodies

There are 7006 matches (from ~500k pages crawled) and after checking a random subset of about 20 all of them were this pattern:

if (document.layers) {
  document.captureEvents(Event.MOUSEDOWN);
  document.onmousedown=clickNS4;
}

This would not break. An exhaustive analysis of all the constants doesn't seem warranted.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d7205bd4386ca9608b0ed138998bcba7d0a3cac

commit 3d7205bd4386ca9608b0ed138998bcba7d0a3cac
Author: foolip <foolip@chromium.org>
Date: Mon Jan 02 18:31:43 2017

Remove legacy constants on Event

Intent to Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/HsAF_yFMvCM/ijFziajpDgAJ

BUG= 674944 
R=tkent@chromium.org
TBR=timvolodine@chromium.org

Review-Url: https://codereview.chromium.org/2579693005
Cr-Commit-Position: refs/heads/master@{#441075}

[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/android_webview/tools/system_webview_shell/test/data/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/fast/dom/constants-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/fast/dom/constants.html
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/fast/xmlhttprequest/xmlhttprequest-get-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt
[delete] https://crrev.com/047fcb2def2217f0b639621c67bc04e6e1088de8/third_party/WebKit/LayoutTests/http/tests/workers/worker-importScriptsOnError-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/imported/wpt/dom/historical-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/platform/win/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/virtual/service-worker-navigation-preload/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/virtual/stable/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-dedicated-worker-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-shared-worker-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-dedicated-worker-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-shared-worker-expected.txt
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/Source/core/events/Event.h
[modify] https://crrev.com/3d7205bd4386ca9608b0ed138998bcba7d0a3cac/third_party/WebKit/Source/core/events/Event.idl

Remove Blink>DOM>Events