[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

Review URL: https://codereview.chromium.org/1226933002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#337624}

CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,lihui7115/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend