Make skia compile in Android GN build.

third_party/expat/BUILD.gn

Index: third_party/expat/BUILD.gn
diff --git a/third_party/expat/BUILD.gn b/third_party/expat/BUILD.gn
index c58b92e4f3fc0ee20c3ebbe76a2c02f1383c4e72..a48d0d0bccf8af85c3559ae3827c41148a4a5672 100644
--- a/third_party/expat/BUILD.gn
+++ b/third_party/expat/BUILD.gn
@@ -2,6 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+if (is_android) {
+  import("//build/config/android/config.gni")
+}
+
 if (is_linux) {
   config("expat_config") {
     libs = [ "expat" ]
@@ -16,6 +20,9 @@ if (is_linux) {
 } else {
   config("expat_config") {
     include_dirs = [ "files/lib" ]
+    if (is_android) {
+      include_dirs += [ "$android_src/external/expat/lib" ]
+    }
     defines = [ "XML_STATIC" ]
   }
 
@@ -27,6 +34,8 @@ if (is_linux) {
       "files/lib/xmltok.c",
     ]
 
+    direct_dependent_configs = [ ":expat_config" ]
+
     defines = [ "_LIB" ]
     if (is_win) {
       defines += [ "COMPILED_FROM_DSP" ]