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The crash component cannot be used on iOS (as it depends on //content).
Turn it into a layered component so that it is possible to put shared
code related to crash there (components/crash_keys and objc_zombie.{h,mm}).

BUG=522955
TBR=sky@chromium.org
TBR=jschuh@chromium.org
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1315303004

Cr-Commit-Position: refs/heads/master@{#349643}