Add missing @SInCE 20.0.

RELNOTES=n/a

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=272289939

Author: cgdecker

android/guava/src/com/google/common/net/HttpHeaders.java

@@ -165,6 +165,8 @@ private ReferrerPolicyValues() {}
   /**
    * The HTTP <a href="https://www.w3.org/TR/service-workers/#update-algorithm">{@code
    * Service-Worker}</a> header field name.
+   *
+   * @since 20.0
    */
   public static final String SERVICE_WORKER = "Service-Worker";
   /** The HTTP {@code TE} header field name. */

guava/src/com/google/common/net/HttpHeaders.java

@@ -165,6 +165,8 @@ private ReferrerPolicyValues() {}
   /**
    * The HTTP <a href="https://www.w3.org/TR/service-workers/#update-algorithm">{@code
    * Service-Worker}</a> header field name.
+   *
+   * @since 20.0
    */
   public static final String SERVICE_WORKER = "Service-Worker";
   /** The HTTP {@code TE} header field name. */