add ccache support to gn builds

build/toolchain/android/BUILD.gn

Index: build/toolchain/android/BUILD.gn
diff --git a/build/toolchain/android/BUILD.gn b/build/toolchain/android/BUILD.gn
index 9ae22183407865cf7c77e65380351f10797ddf8d..78ef8477c44bf0c1abe4833c4f7b2206ec6b6de8 100644
--- a/build/toolchain/android/BUILD.gn
+++ b/build/toolchain/android/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/sysroot.gni")  # Imports android/config.gni.
+import("//build/toolchain/ccache.gni")
 import("//build/toolchain/clang.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/gcc_toolchain.gni")
@@ -36,13 +37,16 @@ template("android_gcc_toolchain") {
     tool_prefix = rebase_path(invoker.tool_prefix, root_build_dir)
 
     if (use_goma) {
-      goma_prefix = "$goma_dir/gomacc "
+      assert(!use_ccache, "Goma and ccache can't be used together.")
+      compiler_prefix = "$goma_dir/gomacc "
+    } else if (use_ccache) {
+      compiler_prefix = "ccache "
     } else {
-      goma_prefix = ""
+      compiler_prefix = ""
     }
 
-    cc = goma_prefix + tool_prefix + "gcc"
-    cxx = goma_prefix + tool_prefix + "g++"
+    cc = compiler_prefix + tool_prefix + "gcc"
+    cxx = compiler_prefix + tool_prefix + "g++"
     ar = tool_prefix + "ar"
     ld = cxx