Add MIPS SIMD Arch (MSA) build flags for GYP/GN builds

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 206 matching lines @@
 | `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)                                     |                                                |
 | `ios_sdk_path`                                  | `ios_sdk_path` and `use_ios_simulator`     | `//build/config/ios/ios_sdk.gni`               |
 | `lsan` (0/1)                                    | `is_lsan` (true/false)                     | `//build/config/sanitizers/sanitizers.gni`     |
 | `mac_sdk_min`                                   | `mac_sdk_min`                              | `//build/config/mac/mac_sdk.gni`               |
 | `mac_sdk_path`                                  | `mac_sdk_path`                             | `//build/config/mac/mac_sdk.gni`               |
 | `mac_sdk`                                       | `mac_sdk_version`                          | `//build/config/mac/mac_sdk.gni`               |
 | `mac_strip_release`                             | `enable_stripping`                         | `//build/config/mac/symbols.gni`               |
 | `mac_want_real_dsym`                            | `enable_dsyms`                             | `//build/config/mac/symbols.gni`               |
+| `mips_msa` (0/1)                                | `mips_use_msa` (true/false)                | `//build/config/mips.gni`                      |

[Dirk Pranke, 2016/07/15 18:59:59]

I wouldn't add this change; we don't list every flag, just the most common ones.

 | `msan` (0/1)                                    | `is_msan` (true/false)                     | `//build/config/sanitizers/sanitizers.gni`     |
 | `SDKROOT` (Mac)                                 | `sysroot`                                  | `//build/config/sysroot.gni`                   |
 | `sysroot`                                       | `sysroot`                                  | `//build/config/sysroot.gni`                   |
 | `target_arch` ("ia32"/"x64"/"arm"/"mipsel")     | `target_cpu` ("x86"/"x64"/"arm"/"mipsel")  | (global)                                       |
 | `toolkit_views` (0/1)                           | `toolkit_views`                            | `//build/config/ui.gni`                        |
 | `tsan` (0/1)                                    | `is_tsan` (true/false)                     | `//build/config/sanitizers/sanitizers.gni`     |
 | `windows_sdk_path`                              | `windows_sdk_path`                         | (internal to `//build/config/win/BUILD.gn`)    |
 
 ### Feature flags
 
@@ skipping 438 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```