Add filters for "gn check"

GN's check command will now read a list of filters from the .gn file. This
allows us to specify a subset of the targets to check so we can incrementally
fix issues while maintaining proper dependencies on good areas.

Adds checking for unused variables in the .gn file to catch misspellings.

Review URL: https://codereview.chromium.org/765633002

Cr-Commit-Position: refs/heads/master@{#306015}

1 files changed, 11 insertions, 2 deletions

diff --git a/.gn b/.gn
index a3bd14d..d1b6e35 100644
--- a/.gn
+++ b/.gn
@@ -1,5 +1,6 @@
-# This file is used by the experimental meta-buildsystem in src/tools/gn to
-# find the root of the source tree and to set startup options.
+# This file is used by the GN meta build system to find the root of the source
+# tree and to set startup options. For documentation on the values set in this
+# file, run "gn help dotfile" at the command line.
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
@@ -8,3 +9,11 @@ buildconfig = "//build/config/BUILDCONFIG.gn"
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.
 secondary_source = "//build/secondary/"
+
+# These are the targets to check headers for by default. The files in targets
+# matching these patterns (see "gn help label_pattern" for format) will have
+# their includes checked for proper dependencies when you run either
+# "gn check" or "gn gen --check".
+check_targets = [
+  "//base"
+]