Specify `run_at:document_start`

[mujurin1]

The following must be added to content_scripts in manifest.json. “run_at”: “document_start” (or “run_at”: “document_end”)

This is not directly related to the use of this extension, but is necessary to solve the following problem

Nico Nico Live Broadcast has a content called “Nico Nico Game”. The NicoNico game runs in Iframe When the extension tries to load scripts or CSS into this Iframe, the NicoLive Game will not start.

The problem seems to depend on specifying when the extension is loaded. The problem seems to occur only when run_at is document_idle. I could not figure out why the problem only occurs if document_idle is run_at.

This problem may not be very common, or it may be a problem on the website side. If there is no problem, please make this change!

(I used the DeelL translation)