Optimized mutator lookup, support list of mutators.

This commit significantly decreases Visualizer/Mutator overhead in debug mode and slightly decreases overhead in fully compiled mode. I was noticing an average replay time* of 50ms per frame for a trace (in debug mode) of the [WebGL Aquarium](https://webglsamples.googlecode.com/hg/aquarium/aquarium.html) before this change and I am now seeing closer to 20ms per frame.

Playback already uses eventIds in a similar way (see [constructCallLookupTable_](https://github.com/google/tracing-framework/blob/63e19623aebdf0d25659ccda9877a3aa5393ebe6/src/wtf/replay/graphics/playback.js#L345)).

\* Replay time is timing frame durations during WebGL playback, without JavaScript/network operations. I am currently working on this feature.