Add "external" dependency mode for Crashpad's GN build.
Add "external" dependency mode for Crashpad's GN build.

This is required for repositories that do not use Crashpad's
third_party/mini_chromium path because they have their own mini_chromium
as a peer to Crashpad.

Bug: crashpad:
Change-Id: I5a765da75fb9efebc4ada17467371d51112fd391
Reviewed-on: https://chromium-review.googlesource.com/1185885
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Commit-Queue: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad