preview mode tab style for pinned tab looks partially hidden behind normal tab

[LaurenWags]

Description

When testing the preview mode tab style for pinned tabs ( #3 from #12565 ) with the right most pinned tab, it looks like the tab hover preview is sort of behind the first normal tab.

Test plan / Steps to Reproduce

1. Have a few pinned tabs and a few not pinned tabs
2. Hover over the not pinned tabs, see the new tab hover style described in the PR from description.
3. Hover over the pinned tabs from left to right.
4. Most of the pinned tabs display the expected tab hover style
5. Hover over the right most pinned tab.

Actual result:
The right side of the pinned tab looks like it's partially behind the normal tab:

Expected result:
Right most pinned tab should look like the other pinned tabs on hover:

Reproduces how often:
Easily

Brave Version

about:brave info:
Brave | 0.22.7
V8 | 6.5.254.41
rev | 8bb7e77
Muon | 5.1.1
OS Release | 16.7.0
Update Channel | Beta
OS Architecture | x64
OS Platform | macOS
Node.js | 7.9.0
Brave Sync | v1.4.2
libchromiumcontent | 65.0.3325.181

Reproducible on current live release:
no

Additional Information

cc @petemill

[LaurenWags]

Verified on macOS 10.12.6 x64 using the following build:

• 0.22.8 3ae27f2
• libchromiumcontent: 65.0.3325.181
• muon: 5.1.2

Verified on Windows 7 x64

• 0.22.8 3ae27f2
• libchromiumcontent: 65.0.3325.181
• muon: 5.1.2

Verified on Ubuntu 17.10 x64 using the following build:

• 0.22.8 3ae27f2
• libchromiumcontent: 65.0.3325.181
• muon: 5.1.2