Issue is reproducible on the latest M-56(56.0.2924.10) of Linux Ubuntu 14.04. Flag is not supported on Mac OS.

Manual Bisect Info:
====================
Good Build:56.0.2905.0 dev
Bad Build:56.0.2906.0 dev

Correction in steps:
======================
(1) Launch chrome and open any page that has scrollbar and observe for scrollbar 
(2) Or hover on scrollbar region to get it activated
(3) When scrollbar vanishes try clicking in scrollbar region to drag/move the scrollbar to top/bottom of page

Actual: Unable to drag the scrollbar to bottom or top as it vanishes 

NOTE: Able to scroll the page using mouse wheel.

Using the per-revision bisect providing the bisect results,
Good build:56.0.2904.0(Revision:428574).
Bad build: 56.0.2906.0 (Revision:428890).

You are probably looking for a change made after 428627 (known good), but no later than 428628 (first known bad).

CHANGELOG URL:
The script might not always return single CL as suspect as some perf builds might get missing due to failure.
 
https://chromium.googlesource.com/chromium/src/+log/e2ba7553b202819e150c10eac65ded657f4df6cf..169f5add2dd64e14cb89c8ac388bc81cc32121d8

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

@bokan - Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner.

Review-Url: https://codereview.chromium.org/2453553003

Thanks!

This is expected behavior, the scrollbar only appears once scrolling is initiated.

++Observation:

Even when focus[When mouse placed on it when it is in activated state]  is on scrollbar it vanishes.I guess this is also a issue.

@bokan:Please provide update on this.

That's tracked as a separate  issue 669668