Pulls the concrete non-test implementations of MemorySnapshot out into a
template. They were effectively identical on Mac and Linux/Android, and
I was going to have to add another identical one for Fuchsia.
Unfortunately it needs to be a template because of the snapshot merging
template it calls that needs the platform-specific ProcessReader (so it
can't just pass in a base ProcessMemory in initialization instead).
This is used on Mac, Linux, Android, and Fuchsia, but there is still a
Windows implementation (different because its ProcessReader is a bit
different) and a test implementation.
Bug: crashpad:196
Change-Id: I4b5575fee0749e96b08e756be1f8380a2c994d7c
Reviewed-on: https://chromium-review.googlesource.com/929308
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
