This CL adds a RequestCrashDumpHandler to request a crash dump over a
socket. Common functionality with LaunchAtCrashHandler is factored out
into a SignalHandler base class.
Bug: crashpad:284
Change-Id: I86293ef599a0dd6eea63c096a5c931c620c05ecc
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1568985
Reviewed-by: Mark Mentovai <mark@chromium.org>