Disable periodic memory dumps when running memory benchmarks
This patch modifies MemoryDumpManager to *not* start periodic memory
dumps when the --enable-memory-benchmarking command-line flag is
present. This is a temporary solution until periodic dumps can be
enabled/disabled via TraceConfig (see https://goo.gl/5Hj3o0).
This change is needed so that telemetry could use on-demand memory dumps
(rather than periodic dumps) to get more stable.
BUG=513692
Review URL: https://codereview.chromium.org/
1260473002
Cr-Commit-Position: refs/heads/master@{#340448}