Let blink scheduler know when background parsing a document

This is necessary to make Virtual time behave deterministically.
Without knowing that a document is being parsed in the background
we might think loading is safe to run timers (which it isn't if
you want determinism).

BUG=546953

Review-Url: https://codereview.chromium.org/2192493002
Cr-Commit-Position: refs/heads/master@{#408668}
10 files changed