build: fix condition for removing NetworkChangeNotifier on hermetic builds

[rogerzanoni]

The base/network_change_notifier_linux.* files are added to the sources only for linux builds, but should be removed for hermetic builds. Add the same is_linux guard to check if the files should be removed from source to fix the following gn gen issue:

ERROR at //net/BUILD.gn:1334:7: Item not found
"base/network_change_notifier_linux.cc"

Bug: 495203133

[github-actions]

🤖 Gemini Suggested Commit Message

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build: Fix condition for removing NetworkChangeNotifier

Previously, the condition for removing network change notifier files
on hermetic builds was applied unconditionally. These files are only
added to the source list for Linux builds. Applying the removal
condition without an is_linux guard could lead to a GN generation
error when attempting to remove files that were never added.

Add an is_linux guard to the removal condition to ensure that the
network change notifier files are only removed if they are present in
the build configuration, preventing 'Item not found' errors.

Bug: 495203133

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[gemini-code-assist]

Code Review

This pull request updates net/BUILD.gn to restrict the removal of Linux-specific network change notifier source files to builds where both is_linux and is_cobalt_hermetic_build are true. I have no feedback to provide.