[api] Add enum type to RAILMode to allow forward-declaration of enum

This allows removing some v8.h includes in blink, and replacing them by
forward declarations.

Change-Id: I3f55669f551e29038918f54a26a0ab032ffb252a
Bug: v8:8788
Reviewed-on: https://chromium-review.googlesource.com/c/1475394Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59630}

include/v8.h

@@ -7055,7 +7055,7 @@ struct JitCodeEvent {
  * See documentation https://developers.google.com/web/tools/chrome-devtools/
  * profile/evaluate-performance/rail
  */
-enum RAILMode {
+enum RAILMode : unsigned {
   // Response performance mode: In this mode very low virtual machine latency
   // is provided. V8 will try to avoid JavaScript execution interruptions.
   // Throughput may be throttled.