The page load metrics component captures and records UMA metrics related to
page loading. This may include page timing metrics, such as time to first
paint, as well as page interaction metrics, such as number of page loads
aborted before first paint.

This component is not supported on iOS, as it requires hooks into Blink for
page timing metrics that are unavailable on iOS.

This component has the following structure:
- browser/: browser process code
- common/: code shared by browser and renderer, such as IPC messages
- renderer/ : renderer process code

For additional information on this component, see 
https://docs.google.com/document/d/1HJsJ5W2H_3qRdqPAUgAEo10AF8gXPTXZLUET4X4_sII/edit