Fix regression in find-in-page behavior with tall blocks.

The refactoring http://crrev.com/19281007/ stopped making use of the
original target point, but this was actually needed to make sure we
don't snap to the top of the block for very tall ones.  This
particularly comes up in the context of find-in-page with tall blocks of
text, which appears not to work at all because of this.

New test coverage WebFrameTest.DivAutoZoomVeryTallTest.

NOTRY=true
BUG=286269,276832

Review URL: https://chromiumcodereview.appspot.com/24364004

git-svn-id: svn://svn.chromium.org/blink/trunk@158205 bbb929c8-8fbe-4397-9dbb-9b2b20218538