Implement ubertracing.

This is based on Nat's patch: https://codereview.chromium.org/441873007/

Move timeline and importers to use telemetry.timeline.TraceData

This (admittedly massive) patch actually gives us an ubertrace object: the
TraceData. The key way to understand this CL is to read timeline/trace_data.py file,
then read importer.py. The rest is cosmetic fallout from the signature changes.

A TraceData contains the data from many tracers, and is always serializable. A
TimelineModel is constructed from a single TraceData. Internally, a
TraceData is an in memory version of json of the following form:
  { traceEvents: ...
    inspectorTimelineEvnets: ...
    tabIds: ...
  }

Since a TraceData has all the tracer data, the model and importer system was
updated so that instead of there being 1 importer per value, the importers
instead indicate which part of the TraceData they can handle, if any.

BUG=356763

Review URL: https://codereview.chromium.org/805463002

Cr-Commit-Position: refs/heads/master@{#309489}

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