content: Fix a typo in GN common/BUILD.gn file.

I got this by change when I ran:
$ gn gen out/Debug_gn --args='os="linux" cpu_arch="arm"'
ERROR at //content/common/BUILD.gn:151:31: Undefined identifier
    if (cpu_arch != "arm" || !is_chromos) {
                             ^---------

BUG=None
TEST=above
TBR=brettw@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/302443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272685 0039d316-1c4b-4281-b951-d872f2087c98

content/common/BUILD.gn

@@ -148,7 +148,7 @@ source_set("common") {
     include_dirs += [ "//third_party/khronos" ]
     configs += [ "//build/config/linux:xcomposite" ]
 
-    if (cpu_arch != "arm" || !is_chromos) {
+    if (cpu_arch != "arm" || !is_chromeos) {
       sources +=[
         "gpu/x_util.cc",
         "gpu/x_util.h",