[tracing] Implement polling in MemoryDumpManager

Implement polling for memory totals when peak triggers are given in
trace config. It also triggers dumps if there is a big increase in
memory total.

BUG=607533

Review-Url: https://codereview.chromium.org/2582453002
Cr-Commit-Position: refs/heads/master@{#451386}
10 files changed