Allow WebRTC to require Mac 10.7 deployments in GN builds.

build_overrides/README.md

 # Build overrides in GN
 
 This directory is used to allow different products to customize settings
 for repos that are DEPS'ed in or shared.
 
 For example: V8 could be built on its own (in a "standalone" configuration),
 and it could be built as part of Chromium. V8 might define a top-level
 target, //v8:d8 (a simple executable), that should only be built in the
 standalone configuration. To figure out whether or not it should be
 in a standalone configuration, v8 can create a file, build_overrides/v8.gni,
 that contains a variable, `build_standalone_d8 = true`.
 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.