[Feature Request]: Emit webContents 'dom-ready' event for sub frames

[samuelmaddock]

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Problem Description

The webContents 'dom-ready' event isn't emitted for any iframe navigation.

electron/shell/browser/api/electron_api_web_contents.cc

Lines 1434 to 1438 in 2be3d03

	void WebContents::DOMContentLoaded(
	content::RenderFrameHost* render_frame_host) {
	if (!render_frame_host->GetParent())
	Emit("dom-ready");
	}

Proposed Solution

Update this event to emit an instance of WebFrameMain

Alternatives Considered

1. Should we instead be consistent with the API and provide isMainFrame, frameProcessId, and frameRoutingId? I think it'd eventually be nice if all navigation events could simply provide a reference to WebFrameMain.

2. Create a new event for sub frames called frame-dom-ready

Additional Information

I'll probably submit a PR for this myself soon enough. Any feedback before that would be appreciated!