https://cs.chromium.org/search/?q=PageLoadTimingMerger::Merge

This approach of updating timing metrics for subframes and the navigation restart penalty is hard to maintain and is already stale where some new metrics have been unintentionally left out.

Possible solutions include:
* Using a type map in the mojo message 
* Using an enum-indexed array in the mojo message that can be looped over to update timing data.