Improve readability and maintainability of nested conditionals in brave_shields_tab_helper.cc (lines 300-330)

[shushannap]

Description

The function in brave-core/browser/brave_shields/brave_shields_tab_helper.cc (lines 300-330) long function with deep nesting using too many if-else statements. This violates the long function and complexity code smells in the guidebooks. This is an issue as it affects maintainability and readability.

Possible fix:

1. Break the function into smaller helper functions to improve modularity.
2. Consider using a mapping structure instead of multiple if-else blocks.