[tracing] Early initialize TracingController for startup tracing

[tracing] Early initialize TracingController for startup tracing

TracingController is required for memory-infra in tracing.
When startup tracing is used in single-process mode, the
TracingController is enabled only on EndStartupTracing(), which
is too late to get any memory-infra dumps in the trace.
This trivial change just anticipates the initialization of
TracingController.
In multi-process mode the TracingController would be initialized
anyways when receiving the first ChildSupportsTracing message.

BUG=532064,524057
TEST=chrome --no-sandbox --single-process --trace-startup=disabled-by-default-memory-infra --trace-startup-duration=4 --trace-startup-file=x.json

Review URL: https://codereview.chromium.org/1345163002

Cr-Commit-Position: refs/heads/master@{#349102}