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

Review URL: https://codereview.chromium.org/145663012

git-svn-id: svn://svn.chromium.org/blink/trunk@166415 bbb929c8-8fbe-4397-9dbb-9b2b20218538