WebGL getVertexAttribOffset should synthesize a GL error on wrong enum values.

Added a missing validation check for the parameter name.

BUG=266296
R=bajones@chromium.org, kbr@chromium.org

Review URL: https://codereview.chromium.org/18914002

git-svn-id: svn://svn.chromium.org/blink/trunk@155242 bbb929c8-8fbe-4397-9dbb-9b2b20218538