[v8 platform] Use V8_DEPRECATED macro for CallOnBackgroundThread().

Follow-up to https://chromium-review.googlesource.com/c/v8/v8/+/941442#message-bc46157bdd4667cb44c2533bfd3201b45c32fe75

TBR=rmcilroy@chromium.org

Bug: v8:7310
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: If3afbdca1393e7f3bb264abadd99f83f5a2b554a
Reviewed-on: https://chromium-review.googlesource.com/942823
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51656}

include/v8-platform.h

@@ -10,6 +10,8 @@
 #include <memory>
 #include <string>
 
+#include "v8config.h"  // NOLINT(build/include)
+
 namespace v8 {
 
 class Isolate;
@@ -323,11 +325,11 @@ class Platform {
    * of tasks wrt order of scheduling, nor is there a guarantee about the
    * thread the task will be run on.
    */
-  virtual void CallOnBackgroundThread(Task* task,
-                                      ExpectedRuntime expected_runtime) {
-    // TODO(gab): Remove this when embedders override CallOnWorkerThread()
-    // instead.
-
+  V8_DEPRECATE_SOON(
+      "ExpectedRuntime is deprecated, use CallOnWorkerThread(Task*) "
+      "instead.",
+      virtual void CallOnBackgroundThread(Task* task,
+                                          ExpectedRuntime expected_runtime)) {
     // An implementation needs to be provided here because this is called by the
     // default implementation below. In practice however, all code either:
     //  - Overrides the new method (thus not making this call) -- i.e. all v8