Include the gaps between pages when searching for a page at a given coordinate.

Previously we weren't including the gap between the pages in the search, which
could result in no page being found.

BUG=303491
R=arv@chromium.org, koz@chromium.org

Review URL: https://codereview.chromium.org/176813012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253705 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253705