Why this matters: If you navigate back to a page with a Scrollable that
has a nonzero scrollOffset, we will restore that scrollOffset. We clamp
the scrollOffset to the contentExtent before the first layout, before
contentExtent is updated to its proper value. Initializing contentExtent
to INFINITY effectively disables the first clamp, until we can get a
valid value from layout. Since the previous scrollOffset was valid, it
seems safe to assume it's still valid.

BUG=https://github.com/flutter/flutter/issues/4883
BUG=https://github.com/flutter/flutter/issues/4797