Make naming of calculateScrollbarModesForLayout reflect it has side effects.

The current name makes it sound like it's just going to calculate the scrollbar
modes. The code also resets the m_viewportRenderer pointer. Update the name to
be calculateScrollbarModesForLayoutAndSetViewportRenderer to make it clearer
there are side effects if using/moving this call.

BUG=

Review URL: https://codereview.chromium.org/148393004

git-svn-id: svn://svn.chromium.org/blink/trunk@166039 bbb929c8-8fbe-4397-9dbb-9b2b20218538