Clean up link references in //tools/gn/README.md.

tools/gn/README.md

Index: tools/gn/README.md
diff --git a/tools/gn/README.md b/tools/gn/README.md
index ad731a6f581cce310b66205b8b4447c0ec482ddb..9c67572506053b8932639e9e95a1b6b61683010a 100644
--- a/tools/gn/README.md
+++ b/tools/gn/README.md
@@ -1,7 +1,7 @@
 # What is GN?
 
 GN is a meta-build system that generates
-[NinjaBuild](https://chromium.googlesource.com/chromium/src/+/master/docs/ninja_build.md)
+[NinjaBuild](https://ninja-build.org)
 files so that you can build Chromium with Ninja.
 
 ## Why did you switch from GYP?
@@ -36,14 +36,14 @@ _As of Oct 2016:_
 
 Read these links:
 
-  * [Quick start](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/quick_start.md)
-  * [FAQ](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/faq.md)
-  * [GYP conversion cookbook](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/cookbook.md)
-  * [Language and operation details](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/language.md)
-  * [Reference](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md) The built-in `gn help` documentation.
-  * [Style guide](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/style_guide.md)
-  * [Cross compiling and toolchains](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/cross_compiles.md)
-  * [Hacking on GN itself](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/hacking.md)
-  * [GNStandalone](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/standalone.md) Standalone GN projects
-  * [UpdateGNBinaries](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/update_binaries.md) Pushing new binaries
-  * [Check](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/check.md) `gn check` command reference
+  * [Quick start](docs/quick_start.md)
+  * [FAQ](docs/faq.md)
+  * [GYP conversion cookbook](docs/cookbook.md)

[brettw, 2016/11/18 21:09:52]

Delete this link to the cookbook.

[Dirk Pranke, 2016/11/18 21:14:28]

Right.

+  * [Language and operation details](docs/language.md)
+  * [Reference](docs/reference.md) The built-in `gn help` documentation.
+  * [Style guide](docs/style_guide.md)
+  * [Cross compiling and toolchains](docs/cross_compiles.md)
+  * [Hacking on GN itself](docs/hacking.md)
+  * [GNStandalone](docs/standalone.md) Standalone GN projects
+  * [UpdateGNBinaries](docs/update_binaries.md) Pushing new binaries
+  * [Check](docs/check.md) `gn check` command reference