Use a single cros_toolchain tarball for ChromeOS build

gyp/common.gypi

Index: gyp/common.gypi
===================================================================
--- gyp/common.gypi	(revision 9724)
+++ gyp/common.gypi	(working copy)
@@ -22,7 +22,8 @@
     'variables': {
       'conditions': [
         [ 'skia_os != OS and not ((skia_os == "ios" and OS == "mac") or \
-                                  (skia_os == "nacl" and OS == "linux"))', {
+                                  (skia_os == "nacl" and OS == "linux") or \
+                                  (skia_os == "chromeos" and OS == "linux"))', {
           'error': '<!(Cannot build with skia_os=<(skia_os) on OS=<(OS))',
         }],
         [ 'skia_mesa and skia_os not in ["mac", "linux"]', {
@@ -40,9 +41,6 @@
         [ 'skia_os == "chromeos" and OS != "linux"', {
           'error': '<!(Skia ChromeOS build is only supported on Linux.)',
         }],
-        [ 'skia_os == "chromeos" and not skia_cros_target', {
-          'error': '<!(skia_cros_target must be defined for ChromeOS)',
-        }],
       ],
     },
     'includes': [