tab icons are not constrained to actual tab space

[alexwykoff]

Test plan

#6900 (comment)

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• Did you search for similar issues before submitting this one?
  yes

• Describe the issue you encountered:
  when opening a private tab then opening the developer's console, it was found that the icons were overhanging the tab.

this also happened when the number of tabs per set was 20

• Platform (Win7, 8, 10? macOS? Linux distro?):
  tested on os x

• Brave Version (revision SHA):
  0.13.0 rc5

• Steps to reproduce:

  1. open the max set of tabs per default set (10)
  2. for the last tab in the set, make it a private tab
  3. open the developer console (fn + shift + f8 on mac)

• Actual result:
  the tab icons extend beyond the tab space

• Expected result:
  tab icons should be constrained to the tab space

• Will the steps above reproduce in a fresh profile? If not what other info can be added?
  yes

• Is this an issue in the currently released version?
  no, the current version is close, but constrained to the tab appropriately

• Can this issue be consistently reproduced?
  yes

• Extra QA steps:
  1.
  2.
  3.

• Screenshot if needed:

• Any related issues: