[api] Deprecate ExternalStringResourceBase::IsCompressible

R=yangguo@chromium.org

Bug: v8:8238
Change-Id: Ia59aefc54c2e9f4fa3348c42fb45e7fadab8ee76
Reviewed-on: https://chromium-review.googlesource.com/c/1349231Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57788}

include/v8.h

@@ -2636,8 +2636,7 @@ class V8_EXPORT String : public Name {
    public:
     virtual ~ExternalStringResourceBase() = default;
 
-    V8_DEPRECATE_SOON("Use IsCacheable().",
-                      virtual bool IsCompressible() const) {
+    V8_DEPRECATED("Use IsCacheable().", virtual bool IsCompressible() const) {
       return false;
     }
 
@@ -2646,16 +2645,7 @@ class V8_EXPORT String : public Name {
      * ExternalStringResource::data() may be cached, otherwise it is not
      * expected to be stable beyond the current top-level task.
      */
-    virtual bool IsCacheable() const {
-#if __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wdeprecated-declarations"
-#endif
-      return !IsCompressible();
-#if __clang__
-#pragma clang diagnostic pop
-#endif
-    }
+    virtual bool IsCacheable() const { return true; }
 
    protected:
     ExternalStringResourceBase() = default;