Currently crashpad's CrashpadHandlerMain can not be overridden. Change
it to be in its own target so that implementers can override it if they
so choose.
Bug: chromium:973167
Change-Id: Ie44eac24c54948154ff550072560732faf621739
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1682932
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Joshua Peraza <jperaza@chromium.org>