Just an FYI: it looks like the left alignment was intentional, based on the comment at
https://chromium.googlesource.com/chromium/src/+blame/978598e8f12159e7842154575396895300a3bd06/chrome/browser/resources/md_extensions/toolbar.html#45

That said, I agree that it looks nicer centered.

They were left-aligned in the mocks, but I have no preference here.  Over to bettes@ to decide.

Centered alignment looks good to me as well. Assuming devmode is mostly used on the main view, I agree the centered alignment looks better. The left-alignment was to attempt to make the actions look decent on both main and detailed views. 

CL at https://chromium-review.googlesource.com/c/chromium/src/+/797935.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e8db293e776b1b6f23317ff20f1a61d432372574

commit e8db293e776b1b6f23317ff20f1a61d432372574
Author: dpapad <dpapad@chromium.org>
Date: Thu Nov 30 17:39:58 2017

MD Extensions polish: Center buttons in dev drawer.

This also allows for some simplifications
 - No longer need to specify --cr-toolbar-field-width.
 - Removed unnecessary dev-controls div wrapper.

Bug:  788981 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ie9425e3eaecde952dff93a7d1af483abc98518e1
Reviewed-on: https://chromium-review.googlesource.com/797935
Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520591}
[modify] https://crrev.com/e8db293e776b1b6f23317ff20f1a61d432372574/chrome/browser/resources/md_extensions/toolbar.html