Allow WebRTC to require Mac 10.7 deployments in GN builds.

build_overrides/README.md

Index: build_overrides/README.md
diff --git a/build_overrides/README.md b/build_overrides/README.md
index d26312c9325ddcc3126ade50802a6cf879454cb1..4b2c71dd555128a31cd2f48f5183f970e558cba4 100644
--- a/build_overrides/README.md
+++ b/build_overrides/README.md
@@ -13,8 +13,17 @@ and import it (as import("//build_overrides/v8.gni") from its top-level
 BUILD.gn file.
 
 Chromium, on the other hand, might not need to build d8, and so it would
-create its own build_overrides/v8.gni file, and in it set 
-`build_standalone_d8 = false`. 
+create its own build_overrides/v8.gni file, and in it set
+`build_standalone_d8 = false`.
 
 The two files should define the same set of variables, but the values can
 vary as appropriate to the needs of the two different builds.
+
+The build.gni file provides a way for projects to override defaults for
+variables used in //build itself (which we want to be shareable between
+projects).
+
+TODO(crbug.com/588513): Ideally //build_overrides and, in particular,
+//build_overrides/build.gni should go away completely in favor of some
+mechanism that can re-use other required files like //.gn, so that we don't
+have to keep requiring projects to create a bunch of different files to use GN.