This is a very simple chrome plugin that allows latex to render in google plus. Installation is (simple). Download the zipped plugin from google drive [1](exactly same, conveniently packaged), go to extensions, check developer mode, unzip to a folder and click Load Unpacked Extenstion to load the folder [1] clickable link is a comment below.

jatexgplus.js

var injectMathJax = document.createElement('script');
var typeSet = document.createElement('script');
var style=document.createElement('style');

//modified: css https://github.com/christianp/mathjax-bookmarklet/blob/master/bookmarklet.js
style.innerText = '.MathJax .mn {background: inherit;} .MathJax .mi {color: inherit;} .MathJax .mo {background: inherit;}';

injectMathJax.type = 'text/javascript';
injectMathJax.src = 'https://c328740.ssl.cf1.rackcdn.com/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML';

typeSet.innerHTML = 'document.body.addEventListener("keydown",function(e){if(e.ctrlKey && e.shiftKey){MathJax.Hub.Queue(["Typeset",MathJax.Hub]); }});document.body.addEventListener("click",function(e) {if(e.target.nodeName == "SPAN"){MathJax.Hub.Queue(["Typeset",MathJax.Hub]); }});';

(document.head||document.documentElement).appendChild(injectMathJax);
(document.head||document.documentElement).appendChild(typeSet);
(document.head||document.documentElement).appendChild(style);

manifest.json

{
  "manifest_version": 2,
  "name": "Mx",
  "description": "math jax inject",
  "version": "1.0",
   "content_security_policy": "script-src 'self' https://c328740.ssl.cf1.rackcdn.com/mathjax/; object-src 'self'",
   "web_accessible_resources":["https://c328740.ssl.cf1.rackcdn.com/mathjax/*"],
   "content_scripts": [{
        "js": ["mx.js"],
        "matches": ["https://plus.google.com/*"]
  	}],
  "permissions": [
    "https://plus.google.com/",  
	"https://c328740.ssl.cf1.rackcdn.com/mathjax/*"
  ]
}
 

https://docs.google.com/uc?export=download&id=0B0XZybAM9XBAeFlkcTRVU1RrcWc