Remove reference to chromeos.gyp.

chromeos.gyp is used here to detect a path to a valid chrome browser
checkout. I'd like to get rid of chromeos.gyp, so this changes the logic
to use BUILD.gn, which was added to the source in June 2014.

BUG=chromium:701617
TEST=None

Change-Id: I16f1705874bb8acafca0d3c38211f5e192e00a33
Reviewed-on: https://chromium-review.googlesource.com/455279
Commit-Ready: Justin TerAvest <teravest@chromium.org>
Tested-by: Justin TerAvest <teravest@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Ting-Yuan Huang <laszio@chromium.org>
diff --git a/build_chrome_browser.py b/build_chrome_browser.py
index 8effa19..e0bbac5 100755
--- a/build_chrome_browser.py
+++ b/build_chrome_browser.py
@@ -131,7 +131,7 @@
 
   if options.cros_workon and not (
       os.path.isdir(options.cros_workon) and os.path.exists(os.path.join(
-          options.cros_workon, 'src/chromeos/chromeos.gyp'))):
+          options.cros_workon, 'src/chromeos/BUILD.gn'))):
     Usage(parser, '--cros_workon must be a valid chromium browser checkout.')
 
   if options.verbose: