Scroll accessibility focused nodes to make them visible.

Since nodes must be (at least partially) visible on screen for
double taps to work when touch exploration is enabled, we scroll the content
whenever accessibility focus is set.  During touch exploration, we delay scrolling
until the pointer goes up to avoid scrolling when the user is actually
exploring the screen.

BUG=335630

Review URL: https://codereview.chromium.org/137533014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247500 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247500