Add a hook to inject trusted Cast roots for testing purposes.

extensions/common/cast/cast_cert_validator.h

Index: extensions/common/cast/cast_cert_validator.h
diff --git a/extensions/common/cast/cast_cert_validator.h b/extensions/common/cast/cast_cert_validator.h
index 8f2f4def340a01456a95867f2a7e819e1db3e081..71ca34c0dddeca48dd0cab08cdf5a2280a0adbc9 100644
--- a/extensions/common/cast/cast_cert_validator.h
+++ b/extensions/common/cast/cast_cert_validator.h
@@ -86,6 +86,16 @@ bool VerifyDeviceCert(const std::vector<std::string>& certs,
 std::unique_ptr<CertVerificationContext> CertVerificationContextImplForTest(
     const base::StringPiece& spki);
 
+// Exposed only for testing, not for use in production code.
+//
+// Injects trusted root certificates into the CastTrustStore.
+// |data| must remain valid and not be mutated throughout the lifetime of
+// the program.
+// Warning: Using this function concurrently with VerifyDeviceCert()
+//          is not thread safe.
+bool AddTrustAnchorForTest(const uint8_t* data,
+                           size_t length) WARN_UNUSED_RESULT;
+
 }  // namespace cast_crypto
 }  // namespace api
 }  // namespace extensions