context capture screenshot: add optional focus top-level context step

CDP's Page.captureScreenshot command[1] is blocked until the active context
gets focus.

[1]: https://chromedevtools.github.io/devtools-protocol/tot/Page/#method-captureScreenshot

Follow-up-of: #183

index.bs

@@ -2175,6 +2175,9 @@ The [=remote end steps=] with <var ignore>session</var> and |command parameters|
    Issue: This ought to be integrated into the update rendering algorithm
    in some more explicit way.
 
+ 1. An implementation may invoke the [=/focusing steps=] on the
+    [=top-level browsing context=] of |context|.
+
  1. Let |root rect| be |document|’s [=document element=]'s [=rectangle=].
 
  1. Let |canvas| be the result of [=trying=] to [=draw a bounding box from the