Fix unintentional revert of SYMROOT change to testing/iossim.

A change made to iossim.gyp in https://chromiumcodereview.appspot.com/14835006
was unintentionally reverted by https://chromiumcodereview.appspot.com/14597007.

Restoring CL 14835006 updates the iossim output path to use $(SYMROOT).

BUG=None
TEST=None

Review URL: https://chromiumcodereview.appspot.com/19985004

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@213010 4ff67af0-8c30-449e-8e8b-ad334ec8d88c