webview: canInvokeDrawGlFunctor should always return true

Now that invokeDrawGlFunctor doesn't use the View anymore, and just
calls a static method on ViewRootImpl. In particular this is required
since invokeDrwaGlFunctor need to still function for views that already
detached.

BUG: 27709981
Change-Id: I2c8c8f4a6943f7eec773265ca709349c5ce0be54