Allow grouping of actions by executed/cached in json profile analysis

[Qinusty]

Description of the problem / feature request:

Currently, the json_profile_merger logic simply takes time spans from the profiles and collates them for analysis.

It would be nice if we could categorise this data based on whether the action was executed locally, executed remotely, cached locally, cached remotely.

Visually it can be quite clear through chrome://tracing whether an action is using the remote cache as it has a span below with "Downloading Outputs", but this isn't considered when collecting this data as the spans are only related via the CPU executed.

Feature requests: what underlying problem are you trying to solve with this feature?

I would like to be able to produce reports on execution time of executed actions from daily CI builds. This is currently not possible as cached actions are grouped in and are quite fast.

[joeleba]

A bit of context regarding json_profile_merge: we're using it to provide an estimated performance breakdown by phases in our daily report: https://perf.bazel.build/bazel/2020/03/12/report.html

We'd be happy to accept PRs on this, provided that it's flag-guarded and doesn't break our dashboard. These tests should be a pretty good safeguard.