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This avoids depending on the name of the C struct. BoringSSL already
provides a header with forward declarations.

Change-Id: I0187f4c51c674746d5926b372ab48ea0e72eae4e
Reviewed-on: https://chromium-review.googlesource.com/c/1287044Reviewed-by: Joe Downing <joedow@chromium.org>
Reviewed-by: Steven Valdez <svaldez@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606923}