Modals are uncentered which leads to clipping when the window is narrow.

[alexwykoff]

Did you search for similar issues before submitting this one?
Yes

Describe the issue you encountered:
When the browser is at minimum width, I attempted to add a bookmark. Because the modal was offset, I was unable to interact properly.

Expected behavior:
The modal should be centered and fit within the minimum window width.

• Platform (Win7, 8, 10? macOS? Linux distro?):
  Tested on OS X
• Brave Version:
  0.11.6 pre-beta6
• Steps to reproduce:
  1. Make the browser window as narrow as possible.
  2. Navigate to a site.
  3. Attempt to bookmark it.
• Screenshot if needed:
  
• Any related issues: