It got broken in r2423.
We were referring to field-name as struct-name.
This patch fixes the regression and also improves the error message
by adding the field-name.

BUG=angle:459
R=kbr@chromium.org

Review URL: https://codereview.appspot.com/12891043

Change-Id: I9ac9a8dafcad1857f7d39e6d494e457d1fc50f70
Reviewed-on: https://chromium-review.googlesource.com/190530Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>