Adds a parameter to the iOS ProcessIntermediateDumps API to allow for
post-crash user stream processing. Also moves user_stream_data_source.h
to handler:common to avoid a dependency cycle on iOS.
This will be used by gwp-asan.
Change-Id: Ic528725276e3c4d284aef89a86cc86878a7af9b0
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/6340388
Reviewed-by: Joshua Peraza <jperaza@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>