[VM/Service] Restructure the timeline flag setting code a bit to make it              convenient for adding a Dart C API function for enabling 	     timeline streams.
[VM/Service] Restructure the timeline flag setting code a bit to make it
             convenient for adding a Dart C API function for enabling
	     timeline streams.

TEST=cq

Change-Id: I885288914e700bfd91605d3d04bba3c0c217656d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/219620
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Commit-Queue: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk