Resolvesbrave/brave-browser#48984
Enabled chromium's mini toolbar instead of our one for SideBySide.
As we have different split view border thickness, mini toolbar's path should be customized.
Also its menu model is replaced with BraveSplitTabMenuModel.
In rounded corners feature, contents shadow is used.
When contents shadow is used, contents container's layer is set.
When contents container has layer, mini urlbar's layer is not visible.
Don't know why and it's difficult to fix.
To fix this issue, hide contents shadow when split view is active.
TEST=SideBySideEnabledBrowserTest.SelectTabTest,
SideBySideWithRoundedCornersTest.ContentsShadowTest
Simon Hong
a9a7fb79ca