Fix for Browser InjectJS when no html.head especially in TamperMonkey and ViolentMonkey

[langren1353]

What: Runtime at browser. fix for less.js is inject into browser by TamperMonkey or ViolentMonkey, but there is no head tag or even html tag.

Why: without head tag, less.js won't work proplywork properly. it will throw a error TypeError: head is undefined

Checklist:

• [ ] Documentation N/A
• [ ] Added/updated unit tests N/A - all test good, and no need for special test
• [x] Code complete

PR Reason for: when ViolentMonkey xx.user.js use less.js as a library, they will inject the script to chrome content, at that time webpage head tag or even html tag doesn't exist, and then error happed, then all xxx.user.js won't work.