Add a GN flag for the Windows multi-dll build.

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") {`                          |
@@ skipping 149 matching lines @@
 declared directly in that file (so other files can't use them). These
 places are noted in the table below.
 
 | *GYP*                                           | *GN*                                       | *GN import*                                    |
 |:------------------------------------------------|:-------------------------------------------|:-----------------------------------------------|
 | `arm_float_abi`                                 | `arm_float_abi`                            | `//build/config/arm.gni`                       |
 | `arm_neon` (0/1)                                | `arm_use_neon` (true/false)                | `//build/config/arm.gni`                       |
 | `arm_neon_optional` (0/1)                       | `arm_optionally_use_neon` (true/false)     | `//build/config/arm.gni`                       |
 | `arm_version`                                   | `arm_version`                              | `//build/config/arm.gni`                       |
 | `asan` (0/1)                                    | `is_asan` (true/false)                     | (global)                                       |
-| `branding` ("Chromium"/"Chrome"")               | `is_chrome_branded` (true/false)           | (global)                                       |
+| `branding` ("Chromium"/"Chrome"")               | `is_chrome_branded` (true/false)           | `//build/config/chrome_build.gni`              |
 | `build_for_tool=="drmemory"`                    | `disable_iterator_debugging` (true/false)  | (internal to `//build/config/BUILD.gn`)        |
 | `build_for_tool=="tsan"`                        | `disable_iterator_debugging` (true/false)  | (internal to `//build/config/BUILD.gn`)        |
-| `buildtype` ("Official"/"Dev")                  | `is_official_build` (true/false)           | (global)                                       |
+| `buildtype` ("Official"/"Dev")                  | `is_official_build` (true/false)           | `//build/config/chrome_build.gni`              |
+| `chrome_multiple_dll` (0/1)                     | `is_multi_dll_chrome` (true/false)         | `//build/config/chrome_build.gni`              |
 | `clang` (0/1)                                   | `is_clang` (true/false)                    | (global)                                       |
 | `clang_use_chrome_plugins` (0/1)                | `clang_use_chrome_plugins` (true/false)    | (internal to `//build/config/clang/BUILD.gn`)  |
 | `component` ("shared_library"/"static_library") | `is_component_build` (true/false)          | (global)                                       |
 | `desktop_linux` (0/1)                           | `is_desktop_linux` (true/false)            | (global)                                       |
 | `disable_glibcxx_debug` (0/1)                   | `disable_iterator_debugging` (true/false)  | (internal to `//build/config/BUILD.gn`)        |
 | `fastbuild` (0/1/2)                             | `symbol_level` (2/1/0 — values inverted)   | (global)                                       |
 | `gomadir`                                       | `goma_dir`                                 | `//build/toolchain/goma.gni`                   |
 | `ios_deployment_target` (string)                | `ios_deployment_target`                    | `//build/config/ios/ios_sdk.gni`               |
 | `GYP_MSVS_OVERRIDE_PATH` environment variable   | `visual_studio_path`                       | `//build/config/win/visual_studio_version.gni` |
 | `GYP_MSVS_VERSION` environment variable         | (none)                                     |                                                |
@@ skipping 458 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```