Improve spacing within SelectMenu

The motivation for this is to align the arrows for the two select menus
found in the Performance panel (cpu/network throttling). The menus are
the same width, but due to its content and the choice of
`justify-content` their arrows did not align.

https://imgur.com/a/jgX1bg3

Bug: None
Change-Id: I1cf7f08c66c99188adc20f984292ce763f628cf8
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6069006
Auto-Submit: Connor Clark <[email protected]>
Reviewed-by: Kateryna Prokopenko <[email protected]>
Reviewed-by: Jack Franklin <[email protected]>
Commit-Queue: Jack Franklin <[email protected]>

Author: Connor Clark

front_end/ui/components/menus/selectMenuButton.css

@@ -42,7 +42,7 @@
 
 #button-label-wrapper {
   display: flex;
-  justify-content: space-around;
+  justify-content: space-between;
   align-items: center;
 }