Annotate unsafe buffer operations in tool_tip_base_view.mm with UNSAFE_BUFFERS macro

This commit replaces the file-level #pragma allow_unsafe_buffers in ui/base/cocoa/tool_tip_base_view.mm with a more precise UNSAFE_BUFFERS() macro annotation. The change only affects array access to the tags parameter in the _removeTrackingRects:count: method, moving from a blanket file-wide unsafe buffer exemption to a single-point annotation with a SAFETY comment noting that count is provided by the caller. This is part of the Chromium security hardening effort (crbug.com/40285824) to incrementally migrate code to safer buffer access patterns.