Use element.getBoundingClientRect() to convert units to pixels

[Comandeer]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

Tests are already in the codebase.

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#5097](https://github.com/ckeditor/ckeditor4/issues/5097): [Chrome] Fixed: Incorrect conversion of points to pixels while using [`CKEDITOR.tools.convertToPx()`](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_tools.html#method-convertToPx).

What changes did you make?

I've switched to using element.getBoundingClientRect() as it returns the current number of pixels. I've also added a fallback to element.clientWidth for IE.

Which issues does your PR resolve?

Closes #5097.

[Comandeer]

I've removed the fallback as we have already CKEDITOR.dom.element#getClientRect() method that does the fallback already.

[jacekbogdanski]

Looks good! I've just added a manual test confirming that borders work properly (sanity check).