Allow dependencies of toolchains in GN.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index bc5288300fa43790180cb7445a37cfcb234c7aad..9ad47a4900ee8b84f68f7aa90852602590e41189 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -21,6 +21,8 @@
 #      The contents of these strings, if specified, will be placed around
 #      the libs section of the linker line. It allows one to inject libraries
 #      at the beginning and end for all targets in a toolchain.
+#  - deps
+#      Just fowarded to the toolchain definition.
 template("gcc_toolchain") {
   toolchain(target_name) {
     assert(defined(invoker.cc), "gcc_toolchain() must specify a \"cc\" value")
@@ -107,5 +109,9 @@ template("gcc_toolchain") {
       cpu_arch = invoker.toolchain_cpu_arch
       os = invoker.toolchain_os
     }
+
+    if (defined(invoker.deps)) {
+      deps = invoker.deps
+    }
   }
 }