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

Turns out FrameView::isScrollable can sometimes crash. Reverting this change
while I try and figure out why.

This reverts commit 1a3edc542e31626a9b46ae2b18eb227c7031f3d6.

TBR=jamesr@chromium.org
BUG=280679

Review URL: https://chromiumcodereview.appspot.com/23477042

git-svn-id: svn://svn.chromium.org/blink/trunk@157381 bbb929c8-8fbe-4397-9dbb-9b2b20218538