Pass the chromeos build variable to isolate.py in _run targets, and update browser_tests.isolate to test it.

This doesn't change the conditional contents of browser_tests.isolate
on any platform. All tests formerly under OS=="linux" are now under
OS=="linux" and (chromeos==0 or chromeos==1).


BUG=158329
TEST=build _run targets, look for new variable in .isolated.state


Review URL: https://chromiumcodereview.appspot.com/12208124

git-svn-id: http://src.chromium.org/svn/trunk/src/build@183668 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

isolate.gypi

@@ -63,6 +63,7 @@
         '--outdir', '<(test_isolation_outdir)',
         '--variable', 'PRODUCT_DIR', '<(PRODUCT_DIR)',
         '--variable', 'OS', '<(OS)',
+        '--variable', 'chromeos', '<(chromeos)',
         '--result', '<@(_outputs)',
         '--isolate', '<(RULE_INPUT_PATH)',
       ],