Add mention of function arguments to base::debug::Alias comments

Change-Id: I40bdae31b55837021cccac92661431da5a161c5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210021Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Oliver Li <olivierli@google.com>
Cr-Commit-Position: refs/heads/master@{#770695}

base/debug/alias.h

@@ -19,12 +19,15 @@ namespace debug {
 // otherwise be live at the point of a potential crash. This can only be done
 // with local variables, not globals, object members, or function return values
 // - these must be copied to locals if you want to ensure they are recorded in
-// crash dumps. Note that if the local variable is a pointer then its value will
-// be retained but the memory that it points to will probably not be saved in
-// the crash dump - by default only stack memory is saved. Therefore the
-// aliasing technique is usually only worthwhile with non-pointer variables. If
-// you have a pointer to an object and you want to retain the object's state you
-// need to copy the object or its fields to local variables.
+// crash dumps. Function arguments are fine to use since the
+// base::debug::Alias() call on them will make sure they are copied to the stack
+// even if they were passed in a register. Note that if the local variable is a
+// pointer then its value will be retained but the memory that it points to will
+// probably not be saved in the crash dump - by default only stack memory is
+// saved. Therefore the aliasing technique is usually only worthwhile with
+// non-pointer variables. If you have a pointer to an object and you want to
+// retain the object's state you need to copy the object or its fields to local
+// variables.
 //
 // Example usage:
 //   int last_error = err_;