domenic@ do you happen to know where in Chrome we define these?
It seems that we are missing this for all the pointerevents as well. But I couldn't find anywhere that has defined this for onclick in the code. 

I'm sorry, I don't. I spent a half-hour spelunking trying to find out to no avail, so I'm very curious with what the eventual answer is!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dcabe3a6d92c6410da0469c2fb02a6b0fd1fe939

commit dcabe3a6d92c6410da0469c2fb02a6b0fd1fe939
Author: nzolghadr <nzolghadr@chromium.org>
Date: Tue Dec 06 19:39:21 2016

Add onauxclick and onpointer* as content attribute

onauxclick and onpointer* attributes were missing
from the content attributes after those features
were added. This CL adds those.

BUG= 670078 

Review-Url: https://codereview.chromium.org/2554433003
Cr-Commit-Position: refs/heads/master@{#436689}

[delete] https://crrev.com/04471f8085185bbdeb796b5539844f8dcf257ba8/third_party/WebKit/LayoutTests/imported/wpt/html/webappapis/scripting/events/event-handler-onauxclick-expected.txt
[modify] https://crrev.com/dcabe3a6d92c6410da0469c2fb02a6b0fd1fe939/third_party/WebKit/Source/core/html/HTMLAttributeNames.in
[modify] https://crrev.com/dcabe3a6d92c6410da0469c2fb02a6b0fd1fe939/third_party/WebKit/Source/core/html/HTMLElement.cpp

Can we merge this into 56?

Your change meets the bar and is auto-approved for M56 (branch: 2924)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/668b73e39142679a1edda6a7415da708ad349269

commit 668b73e39142679a1edda6a7415da708ad349269
Author: Navid Zolghadr <nzolghadr@chromium.org>
Date: Mon Dec 12 16:52:09 2016

Add onauxclick and onpointer* as content attribute

onauxclick and onpointer* attributes were missing
from the content attributes after those features
were added. This CL adds those.

BUG= 670078 

Review-Url: https://codereview.chromium.org/2554433003
Cr-Commit-Position: refs/heads/master@{#436689}
(cherry picked from commit dcabe3a6d92c6410da0469c2fb02a6b0fd1fe939)

Review-Url: https://codereview.chromium.org/2566213002 .
Cr-Commit-Position: refs/branch-heads/2924@{#456}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}

[modify] https://crrev.com/668b73e39142679a1edda6a7415da708ad349269/third_party/WebKit/Source/core/html/HTMLAttributeNames.in
[modify] https://crrev.com/668b73e39142679a1edda6a7415da708ad349269/third_party/WebKit/Source/core/html/HTMLElement.cpp

I just merged this to M56. This should be fixed now in both trunk and 65.