New tab button is not touch screen friendly

[bsclifton]

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

Describe the issue you encountered:
If using a touch screen or when on Windows (which is pickier about right click), the new tab button is much harder to do a long press on. After investigating, it turns out the hitbox is only for the actual SVG itself

Expected behavior:
Clicking should be expanded to the size of the parent element or the height of the tabs area

• Platform (Win7, 8, 10? macOS? Linux distro?):
  all

• Brave Version:
  0.12.10

• Steps to reproduce:

  1. Right click all around the new tab button
  2. Notice that the new tab menu doesn't show up very often, because the hitbox is hard to click

• Screenshot if needed:
  Here's a screenshot on Windows. Notice you can make the system menu show up even when you right click within the bounds of the +. It only seems to show if you DIRECTLY right click / long press on the +
  

• Any related issues:

[luixxiul]

FYI menuButton has the same issue.

[bradleyrichter]

We can add a BG box to these if that helps. Just emulate the main toolbar buttons but the style will need to be different. (not white)

[luixxiul]

button size can be enlarged by increasing the width to ~16px if it's better.

[luixxiul]

I labelled QA/no qa needed as the issue has been superseded with #6125 and PR #6474 for the fix.