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This CL makes following changes:
1. Adds a BACKGROUND mode in the dump triggers in trace config.
1. Periodic dumps can be disabled on passing 0 as interval.
2. On BACKGROUND mode only whitelisted dump providers are invoked.
3. Clean up periodic dump timer set up.
4. MemoryDumpSessionState is initialized with config in the renderer
   also.

BUG=613198

Review-Url: https://codereview.chromium.org/1995573003
Cr-Commit-Position: refs/heads/master@{#396927}