[tracing] Expose the pre-load and merge of ProcessMemoryDump to blink.
This is a follow-up to crrev.com/
1151603004, which introduced the
ability of creating ProcessMemoryDump instances out of band and
later mege them. This CL introduces the wrapper required, on the
chromium side, to expose the same set of features to the blink-side
of MemoryInfra.
This complements the blink-side CL crrev.com/
1145983003
BUG=480500
Review URL: https://codereview.chromium.org/
1150983002
Cr-Commit-Position: refs/heads/master@{#331400}