Move preferred scrollbar style preference change listening from renderer to browser, 2 of 3.

This adds the preference observer to the browser and starts sending the
parameter to the renderer over IPC when it changes. Blink will not use this
until part 3 lands.

BUG=306348

Review URL: https://codereview.chromium.org/136443008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246635 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246635