Set OS=android via GYP_DEFINES

Otherwise, the variable isn't available in all scopes. Instead of playing around
with nested scopes, we just set it in GYP_DEFINES (same as chromium does)

BUG=v8:3405
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/357643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22004 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

Makefile.android

@@ -98,6 +98,7 @@ endif
 
 # For mksnapshot host generation.
 DEFINES += host_os=${HOST_OS}
+DEFINES += OS=android
 
 .SECONDEXPANSION:
 $(ANDROID_BUILDS): $(OUTDIR)/Makefile.$$@

build/android.gypi

@@ -66,7 +66,6 @@
     'android_stlport_library': 'stlport_static',
     # Copy it out one scope.
     'android_webview_build%': '<(android_webview_build)',
-    'OS': 'android',
   },  # variables
   'target_defaults': {
     'defines': [