Using the per-revision bisect providing the bisect results,
Good build:61.0.3141.0(Revision:482153).
Bad build:61.0.3142.0(Revision:482491).

You are probably looking for a change made after 482415 (known good), but no later than 482416 (first known bad).

CHANGE-LOG URL:
---------------
https://chromium.googlesource.com/chromium/src/+log/2daeff3cbf72a9d38a08df1a3093a88ef2151e67..60b41cdfe22237cb667050017349375ca9272ccf

From the CL above, assigning the issue to the concern owner

@sahel : Could you please look into the issue, pardon me if it has nothing to do with your changes and if possible please assign it to concern owner.

Review-Url:https://codereview.chromium.org/2907053004
Note : Windows specific issue and Able to reproduce in latest Canary #61.0.3162.0

 Issue 746209  has been merged into this issue.

 Issue 750396  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a7bd4b63f512849d1107eb4c319835e6fffbd938

commit a7bd4b63f512849d1107eb4c319835e6fffbd938
Author: Sahel Sharify <sahel@chromium.org>
Date: Fri Aug 04 19:12:51 2017

Regression fixed: Autoscrolling on vertical scrollbar works.

Before landing the cl below scrollChain used to get re-computed whenever
needed, including when it wasn't computed for a GSB and needed to get
computed for its following GSU. With the change below now it gets
computed only for GSB events and if it is empty while handling a GSU,
the GSU gets ignored.
https://codereview.chromium.org/2907053004

With this cl GSB events in case of touchpad or auto scrolling are not
handled by scrollbars and the scrollchain is computed while handling
them. Then their following GSU events won't get suppressed.

All/ParameterizedWebFrameTest.scrollbarIsNotHandlingTouchpadScroll/*

Bug:  746922 
Test: fast/events/autoscroll-over-scrollbar.html,
Change-Id: I5620d64044afb3fd0f596267be141f9751626827
Reviewed-on: https://chromium-review.googlesource.com/598999
Reviewed-by: Rick Byers <rbyers@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492095}
[add] https://crrev.com/a7bd4b63f512849d1107eb4c319835e6fffbd938/third_party/WebKit/LayoutTests/fast/events/autoscroll-over-scrollbar.html
[modify] https://crrev.com/a7bd4b63f512849d1107eb4c319835e6fffbd938/third_party/WebKit/Source/core/exported/WebFrameTest.cpp
[modify] https://crrev.com/a7bd4b63f512849d1107eb4c319835e6fffbd938/third_party/WebKit/Source/core/input/ScrollManager.cpp
[modify] https://crrev.com/a7bd4b63f512849d1107eb4c319835e6fffbd938/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

Thanks for the quick fix!

M62 has branched. Should this be considered for a merge?

 Issue 703213  has been merged into this issue.

Sahel, the fix has been baking in trunk for ~1 week, I think it's safe to merge to 61. 

This bug requires manual review: M61 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), ketakid@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Per comment #9, change is baked in Canary and safe to merge. 
But just to be extra cautious, could you please answer followings:
* Does this change having enough automation tests coverage?
* Any other important details to justify the merge.

Please note M61 is already in Beta, so merge bar is VERY high. Thank you.

Re: tests, wheel and scrollbar scrolling have extensive layout tests in Blink so mainline scrolling use cases should be well protected.

I'd say the more compelling case for merging is  issue 746209  which is dup'd into this. That is, scrolls starting over a subscroller's scrollbar won't bubble up to the main page. This can be annoying to users as their scrolls can stop as they slowly scroll down a page.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a57a8b2195e2853a143999a0004278d1e4bd3a32

commit a57a8b2195e2853a143999a0004278d1e4bd3a32
Author: Sahel Sharify <sahel@chromium.org>
Date: Fri Aug 11 20:53:22 2017

Removed the old comment on scrollbars handling GSB's with wheel source.

This is a follow up for https://chromium-review.googlesource.com/598999

With the above change scrollbars don't handle gesture scroll events
while wheel/trackpad/auto scrolling. Scrolling on a scrollbar won't be
any different from scrolling on its corresponding element.

Bug:  746922 
Change-Id: Iefd29840d41e748a73845d73dc72641a137980f3
Reviewed-on: https://chromium-review.googlesource.com/612423
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493846}
[modify] https://crrev.com/a57a8b2195e2853a143999a0004278d1e4bd3a32/third_party/WebKit/Source/core/input/ScrollManager.cpp

Tested the issue on Windows-7 using chrome latest Canary M62-62.0.3185.0 by following steps mentioned in the original comment. Observed that auto scroll when middle clicking on vertical scroll bar with mouse working as expected. Hence adding TE-Verified label.

Please fine the attachment for reference.

Thank you!

Deleted: 746922.mp4 9.5 MB

	746922.mp4 9.5 MB View Download

Approving merge to M61 branch 3163 based on comments #9, #12 and #14. Please merge ASAP. Thank you.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6d4ba5ebece5a23b8abf19c46b7c3a9a4d03121a

commit 6d4ba5ebece5a23b8abf19c46b7c3a9a4d03121a
Author: Sahel Sharify <sahel@chromium.org>
Date: Mon Aug 14 19:23:12 2017

cherry-picked a7bd4b63f512849d1107eb4c319835e6fffbd938 to M61

TBR: bokan@chromium.org,rbyers@chromium.org
Bug:  746922 
Change-Id: I0807c31f67d9d3fc3223753fc028eff53fcc489e
Reviewed-on: https://chromium-review.googlesource.com/614144
Reviewed-by: Sahel Sharifymoghaddam <sahel@chromium.org>
Cr-Commit-Position: refs/branch-heads/3163@{#548}
Cr-Branched-From: ff259bab28b35d242e10186cd63af7ed404fae0d-refs/heads/master@{#488528}
[add] https://crrev.com/6d4ba5ebece5a23b8abf19c46b7c3a9a4d03121a/third_party/WebKit/LayoutTests/fast/events/autoscroll-over-scrollbar.html
[modify] https://crrev.com/6d4ba5ebece5a23b8abf19c46b7c3a9a4d03121a/third_party/WebKit/Source/core/input/ScrollManager.cpp
[modify] https://crrev.com/6d4ba5ebece5a23b8abf19c46b7c3a9a4d03121a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
[modify] https://crrev.com/6d4ba5ebece5a23b8abf19c46b7c3a9a4d03121a/third_party/WebKit/Source/web/tests/WebFrameTest.cpp