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

This reverts commit 3c8dc08b5ef0a09c2e6e7b6a2b9ed85be51577f4.

Original change: https://codereview.chromium.org/40373002

The original change was causing unexplained flake. This suggests that it was not
correct in the first place. Reverting until the flake can be explained/fixed.

TBR=dpranke@chromium.org
BUG=321358

Review URL: https://codereview.chromium.org/76983003

git-svn-id: svn://svn.chromium.org/blink/trunk@162327 bbb929c8-8fbe-4397-9dbb-9b2b20218538