[performance-timeline] merged: getEntries attribute filtering

http://w3c.github.io/performance-timeline/#widl-Performance-getEntries-PerformanceEntryList-FilterOptions-filter

- extends getEntries to accept FilterOptions dictionary (name, entryType,
initiatorType)
- existing getEntriesBy* methods are now defined in terms of getEntries +
FilterOptions

For context, see: https://github.com/w3c/performance-timeline/pull/11.
TL;DR: this update lays the foundation to allow filtering and grouping by
arbitrary attributes - e.g. grouping Server Timing and Resource Timing
records; grouping multi-fetch resources, and so on.

Also, note that we've also clarified that getEntries does *not* need to
return a copy of events:
https://github.com/w3c/performance-timeline/issues/20

ig