This is as a precursor to
https://chromium-review.googlesource.com/374539 which merges
MemorySnapshots and so needs to be able to update them from the minidump
code.
MinidumpMemoryWriter existed to be able to mock for tests; that
behaviour is wrapped up in TestMemorySnapshot now.
BUG=crashpad:61, chromium:638370
Change-Id: I825ec57493b12fc1848018585c14544faa7e66d4
Reviewed-on: https://chromium-review.googlesource.com/374019
Reviewed-by: Mark Mentovai <mark@chromium.org>