[performance-timeline] getEntries filtering & returned object

A couple of updates that should be ready to land:

(1) getEntries will accept optional filter dictionary
-- context:  https://github.com/w3c/performance-timeline/issues/20
-- preview:
https://rawgit.com/w3c/performance-timeline/getEntries-filter/index.html#the-performance-interface


The above explains getEntriesBy{Type, Name} methods in terms of getEntries,
and lays the foundation for us to extend the filter in the future to allow
filtering and grouping for related events and event types - e.g. group
related requests, group resource and server timing events, and so on [1].

(2) getEntries does not need to return a copy of PerformanceEntry objects
-- context: https://github.com/w3c/performance-timeline/issues/20

If you have any comments or suggestions, please ping the associated GH
issues.

ig

[1] https://github.com/w3c/performance-timeline/pull/9#issuecomment-89042906