gn: Mention loadable_module targets in the cookbook.

tools/gn/docs/cookbook.md

 # GYP->GN Conversion Cookbook
 
 [TOC]
 
 ## Targets
 
 | *GYP*                                            | *GN*                                               |
 |:-------------------------------------------------|:---------------------------------------------------|
 | `'type': 'static_library', 'name': 'foo',`       | `static_library("foo") {` or `source_set("foo") {` |
 | `'type': 'shared_library', 'name': 'foo',`       | `shared_library("foo") {`                          |
+| `'type': 'loadable_module', 'name': 'foo',`      | `loadable_module("foo") {`                         |
 | `'type': '<(component)', 'name': 'foo',`         | `component("foo") {`                               |
 | `'type': 'executable', 'name': 'foo',`           | `executable("foo") {`                              |
 | `'type': '<(gtest_target_type)', 'name': 'foo',` | `test("foo") {`                                    |
 | `'type': 'none', 'name': 'foo',`                 | `group("foo") {`                                   |
 
 ### Note on static libraries
 
 A source\_set is basically a transparent static\_library. The source files
 are compiled with the given options but not linked into anything.
 Targets that depend on a source set get the source set's object files
@@ skipping 654 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```