Rework ProcessMemory.Read(Self|Forked) to work without fork()
Rework ProcessMemory.Read(Self|Forked) to work without fork()

Instead of allocating test memory in the parent and then forking and
comparing against it, the child does the allocation and passes back the
region's size and address. Additionally, switch the memcmp()s to be
value-based comparisons instead because the region isn't available in
the parent.

Also renames ProcessMemory.ReadForked to .ReadChild to be correct after
the change from Multiprocess to MultiprocessExec.

This is necessary to have the tests work on Fuchsia.

Bug: crashpad:196, crashpad:215
Change-Id: Id996a21180d87c7f2556283e9f54f6128726f9b8
Reviewed-on: https://chromium-review.googlesource.com/892102
Commit-Queue: Scott Graham <e134406e1863f3b48b0795831afac5c0595efb36@chromium.org>
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Reviewed-by: Joshua Peraza <f5809a5e35ec7e1bd0e25cb0a0b68bb433cf6f17@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad