MathJax: Ignore pagecolor command

This command was not yet supported. This implements it as a noop.
In MathJax it's not really useful to be able to define the background
color as sometimes is required in rendered png mode.

Bug 35186

Change-Id: If6573901bafd8599a01979bcbff89e52b51e4ffd
Merge "MathJax: Ignore pagecolor command"