Support keyboard shortcuts for PDF zooming

### Preflight Checklist

* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem Description
It seems that the only way to zoom in/out of document within PDF viewer is to click the `+` and `-` buttons in the lower right.


It would be nice if user could also use `⌘+` and `⌘-` (or `CTRL+` / `CTRL-` on Windows). These keyboard shortcuts are supported in Chrome. E.g. https://www.fabfilter.com/help/ffproq3-manual.pdf

### Proposed Solution
Implement above-stated keyboard shortcuts for document zooming.

### Alternatives Considered
None.

### Additional Information
I assume it would make sense for this issue to be tagged similarly to these issues: https://github.com/electron/electron/labels/component%2Fpdf-viewer

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I tried enabling page zooming via standard View menu in an Electron app I'm working on (`10.0.0-nightly.20200221`, Mac OS 10.14.6). That did allow for page zooming, but the pdf loaded into an `` tag did not appear to be affected when the embed was focused and I was zooming in/out with keyboard shortcuts. The non-Electron web version of my app (same code minus Electron wrapper) zooms as expected when opened in Chrome.