Add support for user defined "pool" to GN.

tools/gn/docs/reference.md

 # GN Reference
 
 *This page is automatically generated from* `gn help --markdown all`.
 
 ## **\--args**: Specifies build arguments overrides.
 
 ```
   See "gn help buildargs" for an overview of how build arguments work.
 
   Most operations take a build directory. The build arguments are taken
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   Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
          asmflags, defines, include_dirs, ldflags, lib_dirs, libs,
          precompiled_header, precompiled_source
   Deps: data_deps, deps, public_deps
   Dependent configs: all_dependent_configs, public_configs
   General: check_includes, configs, data, inputs, output_name,
            output_extension, public, sources, testonly, visibility
 
 
 ```
+## **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
+  of more than one toolchain it is recommended to specify an explicit
+  toolchain when definining and referencing a pool.
+
+  A pool is referenced by its label just like a target.
+
+```
+
+### **Variables**
+
+```
+  depth*
+  * = required
+
+```
+
+### **Example**
+
+```
+  if (current_toolchain == default_toolchain) {
+    pool("link_pool") {
+      depth = 1
+    }
+  }
+
+  toolchain("toolchain") {
+    tool("link") {
+      command = "..."
+      pool = ":link_pool($default_toolchain)")
+    }
+  }
+
+
+```
 ## **print**: Prints to the console.
 
 ```
   Prints all arguments to the console separated by spaces. A newline is
   automatically appended to the end.
 
   This function is intended for debugging. Note that build files are run
   in parallel so you may get interleaved prints. A buildfile may also
   be executed more than once in parallel in the context of different
   toolchains so the prints from one file may be duplicated or
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           ]
 
         Example for a linker tool that produces a .dll and a .lib. The
         use of {{target_output_name}}, {{output_extension}} and
         {{output_dir}} allows the target to override these values.
           outputs = [
             "{{output_dir}}/{{target_output_name}}{{output_extension}}",
             "{{output_dir}}/{{target_output_name}}.lib",
           ]
 
+    pool [label, optional]
+
+        Label of the pool to use for the tool. Pools are used to limit
+        the number of tasks that can execute concurrently during the
+        build.
+
+        See also "gn help pool".
+
     link_output  [string with substitutions]
     depend_output  [string with substitutions]
     runtime_link_output  [string with substitutions]
         Valid for: "solink" only (optional)
 
         These three files specify which of the outputs from the solink
         tool should be used for linking and dependency tracking. These
         should match entries in the "outputs". If unspecified, the
         first item in the "outputs" array will be used for all. See
         "Separate linking and dependencies for shared libraries"
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 **  \--root**: Explicitly specify source root.
 **  \--runtime-deps-list-file**: Save runtime dependencies for targets in file.
 **  \--script-executable**: Set the executable used to execute scripts.
 **  \--threads**: Specify number of worker threads.
 **  \--time**: Outputs a summary of how long everything took.
 **  \--tracelog**: Writes a Chrome-compatible trace log to the given file.
 **  -v**: Verbose logging.
 **  \--version**: Prints the GN version number and exits.
 
 ```