Extract ContentVerifierKey to its own file.

This removes the necessity of including content_verifier_delegate.h
where only ContentVerifierKey would be required.

Bug: 796395
Change-Id: Icf18f2957291c454c6e8a6971f98be7f9fd2fb78
Reviewed-on: https://chromium-review.googlesource.com/922925
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537363}

• extensions/browser/BUILD.gn[diff]
• extensions/browser/content_verifier/content_hash.h[diff]
• extensions/browser/content_verifier/content_verifier_key.h[Added - diff]
• extensions/browser/content_verifier_delegate.h[diff]