[tracing] Fix MemoryDumpManager periodic scheduler and improve tests

[tracing] Fix MemoryDumpManager periodic scheduler and improve tests

This a cleanup CL in preparation of next changes to memory-infra.
- Add much more coverage to the TraceConfig-related tests as this is
  very critical to avoid regressions in either the chrome://tracing
  UI and telemetry w.r.t. periodic dumps.
- Refactor the tests to use sane and readable in-line expectations
  instead of the awkward back and forths between test fixtures
  and MockDumpProvvider.
- Minor fix in the loogic which schedules periodic dumps in the
  MemoryDumpManager. If ligh_period=1ms and heavy_period=4ms,
  the sequence sholud be HLLLHLLL, not HLLLLHLLLL as it was
  before this change.
- Normalize the "virtual_size" column in the malloc dump provider.

BUG=518823,499731

Review URL: https://codereview.chromium.org/1306843006

Cr-Commit-Position: refs/heads/master@{#347964}