Tab separator is too thick

[petemill]

Consensus from @brave/design and some user feedback is that the 2dp border between tabs is too priminent. Althought that prominence is a feature to aid cognitive understanding of where the different tabs begin and end, it can also add to cognitive load.

[bbondy]

Bumping this to 0.61.x since it'll be included as part of a rebase conflict that will be included with the 73.0.3683.27 upgrade.

[btlechowski]

Verification passed on

Brave	0.61.37 Chromium: 73.0.3683.39 (Official Build) beta (64-bit)
Revision	cc53b0e12fcaf42e4bab8d6c23bd4fb7aae99f6c-refs/branch-heads/3683@{#413}
OS	Windows 7 Service Pack 1 Build 7601.24312

Used test plan from brave/brave-core#1449. The tab separator has 1 px.

top: 0.61.37
bottom: 0.60.45

Verification PASSED on macOS 10.14.3 x64 using the following build:

Brave	0.61.45 Chromium: 73.0.3683.39 (Official Build) beta(64-bit)
Revision	cc53b0e12fcaf42e4bab8d6c23bd4fb7aae99f6c-refs/branch-heads/3683@{#413}
OS	Mac OS X

Verification passed on

Brave	0.61.46 Chromium: 73.0.3683.39 (Official Build) beta (64-bit)
Revision	cc53b0e12fcaf42e4bab8d6c23bd4fb7aae99f6c-refs/branch-heads/3683@{#413}
OS	Linux