Fractional scroll support for main thread document scrolling

This is the first step to make main thread fractional scrolling. This CL
replaces the scroll position and scroll offset in some key steps of document
scrolling from IntPoint/IntSize to DoublePoint/DoubleSize.

BUG=414283

Review URL: https://codereview.chromium.org/595743003

git-svn-id: svn://svn.chromium.org/blink/trunk@183089 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Check for non-finite points before transforming them instead of after.

This avoids potentially passing NaN to clampTo(), which will soon become an
assertion failure.

Along the way, this does a few minor cleanups (use || instead of | for logical
OR, don't nest boolean operators).

BUG=417955
TEST=none

Review URL: https://codereview.chromium.org/605073002

git-svn-id: svn://svn.chromium.org/blink/trunk@183090 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Gardening: styles-add-blank-property.html is flaky

BUG=419531
TBR=eustas@chromium.org,pfeldman@chromium.org

Review URL: https://codereview.chromium.org/618833006

git-svn-id: svn://svn.chromium.org/blink/trunk@183091 bbb929c8-8fbe-4397-9dbb-9b2b20218538