fix: -Wunsafe-buffer-usage warnings in GetNextZoomLevel()

[ckerr]

Description of Change

Fixup to #43803 / e894839 that actually fixes the warning.

Takeaway lesson: give the compiler enough information to know that the array indices are always in the array's valid range.

Warnings fixed by this PR:

2024-10-04T05:18:49.7736915Z ../../electron/shell/browser/ui/inspectable_web_contents.cc:132:46: error: unsafe pointer arithmetic [-Werror,-Wunsafe-buffer-usage]
2024-10-04T05:18:49.7737765Z   132 |       return blink::ZoomFactorToZoomLevel(*--iter);
2024-10-04T05:18:49.7738163Z       |                                              ^~~~
2024-10-04T05:18:49.7738801Z ../../electron/shell/browser/ui/inspectable_web_contents.cc:132:46: note: See //docs/unsafe_buffers.md for help.
2024-10-04T05:18:49.7739875Z ../../electron/shell/browser/ui/inspectable_web_contents.cc:133:19: error: unsafe pointer arithmetic [-Werror,-Wunsafe-buffer-usage]
2024-10-04T05:18:49.7740573Z   133 |     if (!out && ++iter != kEnd)
2024-10-04T05:18:49.7740875Z       |                   ^~~~
2024-10-04T05:18:49.7741546Z ../../electron/shell/browser/ui/inspectable_web_contents.cc:133:19: note: See //docs/unsafe_buffers.md for help.
2024-10-04T05:18:49.7742134Z 2 errors generated.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none