[tracing] MemoryDumpManager delegate for upcoming inter-process dumps
This CL introduces an interface to the MemoryDumpManager to delegate
the generation of dump points. The MDM delegate here is an inversion
of dependency interface, to allow the MDM (which lives in //base) to
coordinate inter-process dumps without having to depend on //ipc and
other layers.
In the upcoming CLs this delegate will be concretely implemented by
TracingControllerImpl (browser side) and ChildTraceMessageFilter
(child process side) to handle the IPC dances.
More context and design doc are available in the attached BUG.
BUG=462930
Review URL: https://codereview.chromium.org/
1042173002
Cr-Commit-Position: refs/heads/master@{#323046}