Add not_needed function

tools/gn/docs/reference.md

 # GN Reference
 
 *This page is automatically generated from* `gn help --markdown all`.
 
 ## Contents
 
 *   [Commands](#commands)
     *   [analyze: Analyze which targets are affected by a list of files.](#analyze)
     *   [args: Display or configure arguments declared by the build.](#args)
     *   [check: Check header dependencies.](#check)
@@ skipping 24 matching lines @@
     *   [declare_args: Declare build arguments.](#declare_args)
     *   [defined: Returns whether an identifier is defined.](#defined)
     *   [exec_script: Synchronously run a script and return the output.](#exec_script)
     *   [foreach: Iterate over a list.](#foreach)
     *   [forward_variables_from: Copies variables from a different scope.](#forward_variables_from)
     *   [get_label_info: Get an attribute from a target's label.](#get_label_info)
     *   [get_path_info: Extract parts of a file or directory name.](#get_path_info)
     *   [get_target_outputs: [file list] Get the list of outputs from a target.](#get_target_outputs)
     *   [getenv: Get an environment variable.](#getenv)
     *   [import: Import a file into the current scope.](#import)
+    *   [not_needed: Mark variables from scope as not needed.](#not_needed)
     *   [pool: Defines a pool object.](#pool)
     *   [print: Prints to the console.](#print)
     *   [process_file_template: Do template expansion over a list of files.](#process_file_template)
     *   [read_file: Read a file into a variable.](#read_file)
     *   [rebase_path: Rebase a file or directory to another location.](#rebase_path)
     *   [set_default_toolchain: Sets the default toolchain name.](#set_default_toolchain)
     *   [set_defaults: Set default values for a target type.](#set_defaults)
     *   [set_sources_assignment_filter: Set a pattern to filter source files.](#set_sources_assignment_filter)
     *   [split_list: Splits a list into N different sub-lists.](#split_list)
     *   [template: Define a template rule.](#template)
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 ```
 
 #### **Examples**
 
 ```
   import("//build/rules/idl_compilation_rule.gni")
 
   # Looks in the current directory.
   import("my_vars.gni")
 ```
+### <a name="not_needed"></a>**not_needed**: Mark variables from scope as not needed.
+
+```
+  not_needed(variable_list_or_star, variable_to_ignore_list = [])
+  not_needed(from_scope, variable_list_or_star,
+             variable_to_ignore_list = [])
+
+  Mark the variables in the current or given scope as not needed, which means
+  you will not get an error about unused variables for these.
+```
+
+#### **Example**
+
+```
+  not_needed("*", [ "config" ])
+  not_needed([ "data_deps", "deps" ])
+  not_needed(invoker, "*", [ "config" ])
+  not_needed(invoker, [ "data_deps", "deps" ])
+```
 ### <a name="pool"></a>**pool**: Defines a pool object.
 
 ```
   Pool objects can be applied to a tool to limit the parallelism of the
   build. This object has a single property "depth" corresponding to
   the number of tasks that may run simultaneously.
 
   As the file containing the pool definition may be executed in the
   context of more than one toolchain it is recommended to specify an
   explicit toolchain when defining and referencing a pool.
@@ skipping 4045 matching lines @@
     *   [--root: Explicitly specify source root.](#--root)
     *   [--runtime-deps-list-file: Save runtime dependencies for targets in file.](#--runtime-deps-list-file)
     *   [--script-executable: Set the executable used to execute scripts.](#--script-executable)
     *   [--threads: Specify number of worker threads.](#--threads)
     *   [--time: Outputs a summary of how long everything took.](#--time)
     *   [--tracelog: Writes a Chrome-compatible trace log to the given file.](#--tracelog)
     *   [-v: Verbose logging.](#-v)
     *   [--version: Prints the GN version number and exits.](#--version)
 ```