[tracing] Fix memory overhead accounting of TraceBufferVector

[tracing] Fix memory overhead accounting of TraceBufferVector

When tracing estimates the memory overhead of its own structs
TraceBufferVector (which itself is just a vector of pointers)
doesn't take into account its own capacity.
This causes sub-accounting when using the "as much as possible"
record strategy, which causes a big upfront reservation.

BUG=495628
Committed: https://crrev.com/954f418961681edbaeb354d59c078b304a863a03
Cr-Commit-Position: refs/heads/master@{#337624}

[Primiano Tucci (use gerrit), 2015-07-07 15:55:39]

primiano@chromium.org changed reviewers:
+ petrcermak@chromium.org

[Primiano Tucci (use gerrit), 2015-07-07 15:55:40]

petrcermak@ PTAL

FYI bashi@ haraken@, this is where the extra malloc things come from when using
telemetry :)

[petrcermak, 2015-07-07 16:00:02]

LGTM.

Thanks,
Petr

[Primiano Tucci (use gerrit), 2015-07-07 16:08:16]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2015-07-07 16:08:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1226933002/1

[commit-bot: I haz the power, 2015-07-07 16:59:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-07 17:01:32]

Patchset 1 (id:??) landed as
https://crrev.com/954f418961681edbaeb354d59c078b304a863a03
Cr-Commit-Position: refs/heads/master@{#337624}