memory-infra: Start disentangling tracing from memory-infra

Starts refactoring the tracing specific code from MemoryDumpManager
into a new class MemoryTracingObserver. This shouldn't change the
behaviour of the MemoryDumpManager.

BUG=703184

Review-Url: https://codereview.chromium.org/2820433005
Cr-Commit-Position: refs/heads/master@{#466936}
7 files changed