Add timing data for entried with missing timings key

@jzoldak Please review... 

Note that when these values are used, the it uses max(val, 0). 

Example:
```
dns_time = self.entry["timings"]["dns"]
return max(dns_time, 0)
```
Since we know that this is specific to BrowserMob Proxy, can we look for "name":"BrowserMob Proxy" instead and then apply the fix? 

Or maybe you worried about performance? If so maybe look for the -999 status within a new workaround_for_browsermob_proxy method and then only iterate through the entries if it exists.