JSRuntime lifetime managed by WebFPage lifetime

[david-clang]

Prior to WebF 0.16.0 "Add Dedicated Threading support.", the JSRuntime lifecycle was managed by the DartIsolateContext lifecycle.

But in Dedicated threads, the JSRuntime life cycle must be managed by the WebFPage life cycle, and the runtime_ redundancy created in DartIsolateContext.

Therefore, JSRuntime lifetime managed by WebFPage lifetime in UI thread mode is more uniform