Tiny icons on high DPI screens

### Prerequisites

* [x] Put an X between the brackets on this line if you have done all of the following:
* Reproduced the problem in Safe Mode:
* Followed all applicable steps in the debugging guide:
* Checked the FAQs on the message board for common solutions:
* Checked that your issue isn't already filed:
* Checked that there is not already an Atom package that provides the described functionality:

### Description

When using a high DPI screen (sometimes known as "Retina" display), the icon will become unusably tiny. Screenshot from `tool-bar-main`:

### Steps to Reproduce

1. Buy a high DPI screen :clown:
2. Install any package consuming the `tool-bar` service
3. View result

**Expected behavior:**

Screenshot from `tool-bar-main` with the fix from the PR applied.

**Actual behavior:**

Screenshot from `tool-bar-main`, without the fix

**Reproduces how often:**

100%

### Versions

Atom : 1.60.0
Electron: 9.4.4
Chrome : 83.0.4103.122
Node : 12.14.1

Start by reproducing the high-DPI behavior in a package consuming the tool-bar service, using the tool-bar-main example named in the issue. Trace the toolbar icon styling and compare the current result with the expected screenshot; done means icons are no longer unusably tiny on high-DPI screens.

Written by the indexing model from the issue text.