Memory overhead from collection session measurements and subsession measurements

[Vladan Djeric]

We're considering submitting both a full-session Telemetry/FHR ping and
subsession pings during each Firefox session as a stopgap measure while we
unify Telemetry & FHR.

I volunteered to look into the memory overhead of this approach.

In my Firefox 35 (release) session on Windows 7 with an uptime of 18 hours,
I noted the following entries for Telemetry in about:memory:

   - 1MB TelemetryFile for single-process
      - This is completely unexpected, I filed bug 1126986
      <https://bugzilla.mozilla.org/show_bug.cgi?id=1126986>
   - *270 kB for Telemetry heap data allocated by C++ code*
      - *Pretty much all the Telemetry data (histograms and other data
      types) are stored in C++ code*
      - 106 kB for TelemetryPing.jsm
   - 74 kB for BrowserUITelemetry.jsm
   - 34 kB for TelemetryLog.jsm
   - 34 kB for TelemetryStopwatch.jsm
   - 29 kB for TelemetryTimestamps.jsm
   - 27 kB for UITelemetry.jsm
   - 23KB for TelemetryStartup.js

The actual Telemetry ping for this session was 112 KB in uncompressed JSON
form.
An 8 day session from Nightly 38 (non-E10S) on Windows 8.1 had very similar
numbers.
Each process in an E10S session had roughly the same cost as the numbers
above.

I think an additional ~300KB memory overhead on desktop machines is
acceptable.
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