[vm, fuchsia] Allow for enabling trace categories through both traceu…

…til and the service protocol.

This is needed so that we can make progress on using flutter drive tests on Fuchsia.

Change-Id: Ifea0e4747c0cf28b85ad5a2eff20bdf361a10be3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/104948
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/timeline.h

@@ -60,10 +60,14 @@ class TimelineStream {
 
   bool enabled() {
 #if defined(HOST_OS_FUCHSIA) && !defined(FUCHSIA_SDK)
+#ifdef PRODUCT
     return trace_is_category_enabled(fuchsia_name_);
+#else
+    return trace_is_category_enabled(fuchsia_name_) || enabled_ != 0;
+#endif  // PRODUCT
 #else
     return enabled_ != 0;
-#endif
+#endif  // defined(HOST_OS_FUCHSIA) && !defined(FUCHSIA_SDK)
   }
 
   void set_enabled(bool enabled) { enabled_ = enabled ? 1 : 0; }