Rework ReadCString[SizeLimited](Self|Forked) to not use fork()
Rework ReadCString[SizeLimited](Self|Forked) to not use fork()

Instead of using pointers shared between the parent/child due to fork,
explicitly builds and passes them between processes. This is
unfortunately a bit more verbose, but seems like it tests functionality
a little better, and is required to have the test work on Fuchsia.

Also renames the ...Forked to ...Child to be correct after the change
from Multiprocess to MultiprocessExec.

Bug: crashpad:196, crashpad:215
Change-Id: I610a7f1e35b6513805c27d9e610f7a9b9820cabc
Reviewed-on: https://chromium-review.googlesource.com/892286
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