[tracing] Increase rate of MemoryInfra dumps and throttle mmaps
As a short-term improvement of the memory profiling in tracing,
this CL is increasing the frequency of memory dumps and throttling
down the rate of mmap dumps (which are really heavy and expensive).
Furthermore this CL adds some small performance optimization to the
memory dumping fast paths and fixes a double accounting issue in the
TraceEventMemoryOverhead class.
BUG=495628
Review URL: https://codereview.chromium.org/
1177423002
Cr-Commit-Position: refs/heads/master@{#334230}