Issues with WEBGL_debug extension spec

A couple of issues with this extension specification came up during a recent face-to-face meeting:
- The MAX_DEBUG_MESSAGE_LENGTH_KHR and MAX_DEBUG_GROUP_STACK_DEPTH_KHR enums should be removed since they're unnecessary (the extension just returns strings to JavaScript).
- The extension should mention that DEBUG_OUTPUT_SYNCHRONOUS was removed. This eliminates undefined behavior from reentrant debug callbacks.

Also, hopefully there won't be issues with needing programs' line numbers to be the first thing on the line, etc. (as with getProgramInfoLog in the past).

Start by locating the WEBGL_debug extension specification and review the definitions of MAX_DEBUG_MESSAGE_LENGTH_KHR, MAX_DEBUG_GROUP_STACK_DEPTH_KHR, and DEBUG_OUTPUT_SYNCHRONOUS. Remove the unnecessary enums, document that DEBUG_OUTPUT_SYNCHRONOUS was removed, and verify the specification addresses the concern about program line-number formatting.

Written by the indexing model from the issue text.