Clear profiling data for root when the root is unmounted

See PR #230

We currently only reset profiling data when profiling is re-started (in onProfilingStatus)

react-devtools-experimental/src/devtools/store.js

Lines 965 to 972 in c28acc5

    
           onProfilingStatus = (isProfiling: boolean) => { 
        
             if (isProfiling) { 
        
               this._importedProfilingData = null; 
        
               this._profilingOperations = new Map(); 
        
               this._profilingScreenshots = new Map(); 
        
               this._profilingSnapshot = new Map(); 
        
               this.roots.forEach(this._takeProfilingSnapshotRecursive); 
        
             }

We should probably also clear it out if the root it corresponds to gets unmounted.

This should be easy for _profilingOperations and _profilingSnapshot since they are just maps of root ID to data.

However, I think _profilingScreenshots is actually currently broken because it maps commit index to screenshot, rather than root id to commit index to screenshot. So we should fix that too!

A basic test or two that verifies this would be nicer. 😄

	onProfilingStatus = (isProfiling: boolean) => {
	if (isProfiling) {
	this._importedProfilingData = null;
	this._profilingOperations = new Map();
	this._profilingScreenshots = new Map();
	this._profilingSnapshot = new Map();
	this.roots.forEach(this._takeProfilingSnapshotRecursive);
	}