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Currently there is a virtual time deadlock between HTML imports and
PendingScript. I'm not sure about modules, but at least classic
PendingScript is effected.

* PendingScript will keep virtual time paused until it's ready to be
  executed (Until the document is not script-blocking).
* Script execution will be paused on the document until the HTML import
  has finished.
* The HTML import will not finish unless virtual time is unpaused
  (It fires an immediate timer to update it's state)

The deadlock happens because timers (even immediate ones) will not fire
if they were scheduled after virtual time was paused.

To resolve this this CL unpauses PendingScript's virtual time pauser
earlier; once the script is marked "ready". This means that
PendingScript will unpause virtual time before waiting for HTML import
to finish, and the deadlock is resolved.

Change-Id: I1763229bd73096000bc461c678b061a345c3b530
Reviewed-on: https://chromium-review.googlesource.com/1104314Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Zoe Clifford <zoeclifford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569630}