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Layout changes trigger view changes which get sent to the plugin. With the
in process plugin (PDF), the plugin is notified synchronously of the view
change. It then might execute scripts synchronously in the plugin but
scripts are not meant to be executed during layout changes. This change
runs the scripts asynchronously. I tested print preview to ensure that it
still works correctly.

Note that once we remove in-process plugins this won't be an issue because
view changes happen asynchronously out of process (besides the fact that
synchronous script execution is limited to private plugins).

BUG=351636

Review URL: https://codereview.chromium.org/530363002

Cr-Commit-Position: refs/heads/master@{#293256}