clock widget shouldn't shift when showing/hiding Brave Rewards widget

[rebron]

Description

Hiding the Brave Rewards widget shifts the clock widget right. Showing the Brave Rewards widget centers the clock to the Brave Rewards widget.

Clock should not shift.

Steps to Reproduce

1. Go to brave://newtab
2. Toggle Show/Hide Brave Rewards widget
3. Notice clock shifts when Brave Rewards widget is toggled.

Actual result:

Expected result:

Clock should stay in the same position as the first screenshot above regardless of whether Brave Rewards widget is shown or hidden.

Brave version (brave://version info)

Version/Channel Information:

1.5.115 and above.

[rebron]

cc: @karenkliu