Not being a WhatsApp user I can't test this directly.  Can you take a video?

it's just twitching about 1 pixel left to right when I receive a message.
The tab title shows a (1) and also the speaker icon appears briefly because
WhatsApp Web does make a short notification sound when you receive a
message.

Ok the speaker/audio icon appearing and disappearing is definitely the cause. When it appears, the favicon moves to the right 1 or 2 pixels, and when the speaker icon disappears, the WhatsApp favicon moves left, back to its original position. 

Sorry for so many posts, I just forgot to add that it's not just the favicon, the tab title twitches/shifts along with it. And of course it is not specific to WhatsApp Web (sorry for specifying WhatsApp Web in the title, that was a mistake), try pausing and playing any YouTube video to repro https://www.youtube.com/watch?v=yCSz1pPuOpA 

Able to reproduce issue on reported chrome version 67.0.3396.62  & on latest chrome 69.0.3446.0 using Windows 10,Ubuntu 14.04 and Mac 10.13.3. Hence providing bisect information below.

Bisect Info:
================
Good build: 66.0.3345.0
Bad build: 66.0.3347.0

You are probably looking for a change made after 536161 (known good), but no later than 536191 (first known bad).

CHANGELOG URL:
https://chromium.googlesource.com/chromium/src/+log/df8e815b77e073d32ff6b922bdac70c9b1c1337a..056d08eee04db3f48c53adb416c7044993ebceab

suspect: 
https://chromium.googlesource.com/chromium/src/+/14f736d1b6aefdf1dfe01d1cbd99433a8ac86e62

Reviewed-on: https://chromium-review.googlesource.com/898532

@malaykeshav: Please confirm the issue and help in re-assigning if it is not related to your change.Adding RB-Stable as this is recent regression, Please remove if not the case.

Thanks!

The regression happened due to the removal of the usage of IconCapacity in a tab.
Earlier we used to add an extra padding if the icon capacity of the tab was atleast 3, that is if the tab was big enough to show the favicon, alert icon and close button.

However, instead of using capacity, we now set the visibility for each of the 3 icon based on heuristics similar to what was in IconCapacity. This means that even if the tab has the capacity for 3 icons, we will not add the extra padding until all 3 icons are visible.
In the above case, all 3 icons are visible when there is a notification and we show the third speaker icon.

We need to set |extra_padding_before_content_| based on capacity instead of actual visibility. Can by done by simply checking if sum of icon width + padding is less than available width.
https://chromium-review.googlesource.com/c/chromium/src/+/898532/32/chrome/browser/ui/views/tabs/tab.cc#1140

@pkasting WDYT?

I don't have time to think deeply but that sounds fine at a distance and it seems like you've thought reasonably deeply about this.  Send me a CL?

Also note https://chromium.googlesource.com/chromium/src/+/2c622366bc98d65ee93cfe0b8a57a95cd9b8df39 which touched the code in here after your change.

Malay, feel free to work with bsep@ on getting this bug fixed -- it's a pretty important regression IMO, and with the tabstrip changes in refresh people will be more aware of tab-area regressions.