[tracing] Expose memory-infra to Blink (chromium-side changes).

[tracing] Expose memory-infra to Blink (chromium-side changes).

This complements crrev.com/1111383002 (blink side) and exposes the
memory-infra to Blink. The same logic and class taxonomy applies.
This CL introduces the wrapping classes to expose MemoryDumpProvider,
ProcessMemoryDump and MemoryAllocatorDump to Blink's platform.
For more documentation see the design doc: http://goo.gl/Bdyql6

BUG=480500

Review URL: https://codereview.chromium.org/1108403006

Cr-Commit-Position: refs/heads/master@{#328938}