[cr144] Use UNSAFE_TODO for chromium_src override.

The upstream code we override uses UNSAFE_TODO, so we'll match for now.

In file included from ../../brave/chromium_src/chrome/installer/util/shell_util.cc:72:
../..\chrome/installer/util/shell_util.cc(484,5): error: unsafe buffer access [-Werror,-Wunsafe-buffer-usage]
  484 |     BRAVE_GET_SHELL_INTEGRATION_ENTRIES
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../brave/chromium_src/chrome/installer/util/shell_util.cc(59,33): note: expanded from macro 'BRAVE_GET_SHELL_INTEGRATION_ENTRIES'
   59 |   const std::wstring file_ext = ShellUtil::kPotentialFileAssociations[i]; \
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../..\chrome/installer/util/shell_util.cc(484,5): note: See //docs/unsafe_buffers.md for help.
../../brave/chromium_src/chrome/installer/util/shell_util.cc(59,33): note: expanded from macro 'BRAVE_GET_SHELL_INTEGRATION_ENTRIES'
   59 |   const std::wstring file_ext = ShellUtil::kPotentialFileAssociations[i]; \
      |                                 ^
1 error generated.

Chromium change:

https://source.chromium.org/chromium/chromium/src/+/ce967e366f66cb46d2c8b8219fa2972ed9d64762

commit ce967e366f66cb46d2c8b8219fa2972ed9d64762
Author: Arthur Sonzogni <arthursonzogni@chromium.org>
Date:   Thu Nov 20 04:35:05 2025 -0800

    Convert to UNSAFE_TODO in chrome

    This is an automated #cleanup patch using the [Script] below.

    We are migrating from coarse-grained file-level suppression (#pragma
    allow_unsafe_buffers) to granular, expression-level markers
    (UNSAFE_TODO()). The pragma disables safety checks for an entire file,
    whereas UNSAFE_TODO() isolates specific potentially unsafe operations,
    allowing the rest of the file to be enforced as safe.

    This CL was uploaded by an experimental version of git cl split
    (https://crbug.com/389069356).

    Script: https://docs.google.com/document/d/1ORQGBNn2R-CEvNbDTjRd-GrOBOFlCxIHdcvSUA_EhR4/edit?usp=sharing
    AX-Relnotes: N/A
    Cleanup: This is an automated #cleanup.
    Bug: 409340989

chromium_src/chrome/installer/util/shell_util.cc

@@ -55,13 +55,14 @@ int GetIconIndexForFileType() {
 
 // Give BraveXXFile prog id for some file type.(ex, .pdf or .svg) instead of
 // BraveHTML.
-#define BRAVE_GET_SHELL_INTEGRATION_ENTRIES                               \
-  const std::wstring file_ext = ShellUtil::kPotentialFileAssociations[i]; \
-  if (installer::ShouldUseFileTypeProgId(file_ext)) {                     \
-    entries->push_back(std::make_unique<RegistryEntry>(                   \
-        capabilities + L"\\FileAssociations", file_ext,                   \
-        installer::GetProgIdForFileType()));                              \
-    continue;                                                             \
+#define BRAVE_GET_SHELL_INTEGRATION_ENTRIES                  \
+  UNSAFE_TODO(const std::wstring file_ext =                  \
+                  ShellUtil::kPotentialFileAssociations[i];) \
+  if (installer::ShouldUseFileTypeProgId(file_ext)) {        \
+    entries->push_back(std::make_unique<RegistryEntry>(      \
+        capabilities + L"\\FileAssociations", file_ext,      \
+        installer::GetProgIdForFileType()));                 \
+    continue;                                                \
   }
 #else
 #define BRAVE_GET_CHROME_PROG_ID_ENTRIES