Sync gflags into the right location.

The DEPS entry for gflags added in https://codereview.chromium.org/1413723002/
assumed the Chromium/WebRTC checkout layout with 'src' as the
top-directory name. That doesn't work for libyuv, since the top-directory
is named 'libyuv'.

BUG=libyuv:507

Review URL: https://codereview.chromium.org/1410673004 .

DEPS

@@ -13,7 +13,7 @@ vars = {
 # NOTE: Prefer revision numbers to tags for svn deps. Use http rather than
 # https; the latter can cause problems for users behind proxies.
 deps = {
-  'src/third_party/gflags/src':
+  Var('root_dir') + '/third_party/gflags/src':
     Var('chromium_git') + '/external/gflags/src@e7390f9185c75f8d902c05ed7d20bb94eb914d0c', # from svn revision 82
 }