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Updates various files with to conform to iOS13 SDK headers changes:
  Nullability and availability changes
  Updates security types
  Updated default values (modalPresentationStyle)
  Updates objc_zombie with IMP changes (as well as adding the accidentally-omitted tests)

TBR=rohitrao

Change-Id: I7d88a8c0fbb764484050be67dd33adcd11d343dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642508
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666142}