Fix the amusingly wrong '2 arguments required, but only 2 present' error message.

http://src.chromium.org/viewvc/blink?revision=160196&view=revision introduced
improvements to the "Not enough arguments" error message thrown from bindings.
I typo'd the expected number of arguments for one of the WebGL helper functions
however, which lead to amusingly incorrect results.

This patch adds the missing bit to the character I typed, and adds a test to
ensure I got things right.

BUG=323317

Review URL: https://codereview.chromium.org/87753002

git-svn-id: svn://svn.chromium.org/blink/trunk@162686 bbb929c8-8fbe-4397-9dbb-9b2b20218538