Revert of Remove the layout timer (https://codereview.chromium.org/145663012/)

Reason for revert:
Broke browser_tests:DeclarativeContentApiTest.Overview & content_browsertests:SiteIsolationPolicyBrowserTest.CrossSiteDocumentBlockingForDifferentTargets.

Original issue's description:
> Remove the layout timer
> 
> As discussed on blink-dev, we want to remove the layout timer
> in order to better synchronize work with the compositor.
> Prior to this CL, we'd trigger layout at arbitrary times,
> which meant we could end up doing layout multiple times per
> frame unnecessarily.
> 
> BUG=340357
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166415

TBR=ojan@chromium.org,eseidel@chromium.org,abarth@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=340357

Review URL: https://codereview.chromium.org/154273002

git-svn-id: svn://svn.chromium.org/blink/trunk@166433 bbb929c8-8fbe-4397-9dbb-9b2b20218538