[tracing] Introduce MemoryDumpSessionState
MemoryDumpSessionState is a container class to hold objects that need
to be shared between all the ProcessMemoryDumps for all the lifetime
of a tracing session.
A concrete use case is going to be represented by the dictionary of
allocator attributes types, that will be dumped once by the
MemoryDumpProviders into the MemoryDumpSessionState when the trace
starts and will be accessed when generating the final trace data.
BUG=466141
Review URL: https://codereview.chromium.org/
1093953002
Cr-Commit-Position: refs/heads/master@{#325987}