Ignore should-scroll-on-main-thread if main frame is not scrollable

This change had been reverted previously because of LayoutTest flakiness. I had
been concerned that this was because the code was actually incorrect, but it
looks like the real problem is that we're not correctly resetting between
layout tests inside Internals::resetToConsistentState. I've updated that fn
and the tests are no longer flaky.

This reverts commit 757a502a1b21193db4b84db6cff0703c112291ef.
Review URL: https://codereview.chromium.org/76983003

BUG=321358
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/85763003

git-svn-id: svn://svn.chromium.org/blink/trunk@162663 bbb929c8-8fbe-4397-9dbb-9b2b20218538