prepare skia for shared library build on android

gyp/common.gypi

Index: gyp/common.gypi
diff --git a/gyp/common.gypi b/gyp/common.gypi
index d3e1498efcb11233f0ef3b9ed2cf6f358d30bc8b..2be8bb8142500c71cd10f51b86b2fa663ea3c202 100644
--- a/gyp/common.gypi
+++ b/gyp/common.gypi
@@ -39,6 +39,15 @@
         }],
       ],
     },
+    # Validate the 'skia_os' setting against 'skia_shared_lib', because shared
+    # library build is only supported on Android.
+    'variables': {
+      'conditions': [
+        [ 'skia_os != "android" and skia_shared_lib', {
+          'error': '<!(Skia shared lib build only currently supported on Android.)',
+        }],
+      ],
+    },
     'includes': [
       'common_conditions.gypi',
     ],