Teach cr to set the correct target os for chromeos when generating with gn.

BUG=None

Review URL: https://codereview.chromium.org/1128843008

Cr-Commit-Position: refs/heads/master@{#330515}

tools/cr/cr/base/linux_chromeos.py

@@ -15,6 +15,7 @@ class LinuxChromeOSPlatform(cr.Platform):
       CR_BINARY=os.path.join('{CR_BUILD_DIR}', '{CR_BUILD_TARGET}'),
       CHROME_DEVEL_SANDBOX='/usr/local/sbin/chrome-devel-sandbox',
       GYP_DEF_chromeos=1,
+      GN_ARG_target_os='"chromeos"',
   )
 
   @property