[unsafe-buffers] Fix UNSAFE_TODO in BraveStatsUpdaterBrowserTest (#28718)

The simpler fix here is to avoid any buffer operation.

Resolves https://github.com/brave/brave-browser/issues/45513

browser/brave_stats/brave_stats_updater_browsertest.cc

@@ -160,9 +160,7 @@ IN_PROC_BROWSER_TEST_F(BraveStatsUpdaterBrowserTest,
   WaitForReferralInitializeCallback();
   WaitForStandardStatsUpdatedCallback();
 
-  // We get //1/usage/brave-core here, so ignore the first slash.
-  EXPECT_STREQ(UNSAFE_TODO(GetUpdateURL().path().c_str() + 1),
-               "/1/usage/brave-core");
+  EXPECT_EQ(GetUpdateURL().path(), "//1/usage/brave-core");
 
   // First check preference should now be true
   EXPECT_TRUE(g_browser_process->local_state()->GetBoolean(kFirstCheckMade));