MeterRole was missed in computeAccessibilityIgnored()

In this CL https://codereview.chromium.org/652663003/ MeterRole is not added to
computeAccessibilityIgnored().
So, adding that change in this CL.

BUG=421376

Review URL: https://codereview.chromium.org/653283003

git-svn-id: svn://svn.chromium.org/blink/trunk@183778 bbb929c8-8fbe-4397-9dbb-9b2b20218538