Improve Windows GN build setup.

tools/gn/build_settings.h

Index: tools/gn/build_settings.h
diff --git a/tools/gn/build_settings.h b/tools/gn/build_settings.h
index fc175dfe795167204e97b678c664a4b9df8bb93a..231d0409da3ba20e65acddbdf2cf71283a90b9ba 100644
--- a/tools/gn/build_settings.h
+++ b/tools/gn/build_settings.h
@@ -60,7 +60,8 @@ class BuildSettings {
 
   // The build directory is the root of all output files. The default toolchain
   // files go into here, and non-default toolchains will have separate
-  // toolchain-specific root directories inside this.
+  // toolchain-specific root directories inside this. This should be set after
+  // the root path for the proper directory to be created.
   const SourceDir& build_dir() const { return build_dir_; }
   void SetBuildDir(const SourceDir& dir);