We were checking readTarget twice, instead of both read and write.

BUG=angle:800

Change-Id: I4096a9c4e0fdec9c77bfc25250923c188d2b43db
Reviewed-on: https://chromium-review.googlesource.com/224102Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>