This patch changes the semantics of tileWidth/Height to include the border region, and
uses an offset to take into account the fact that there is no outer border for outer
tiles. This patch also fixes a previous bug where the right column and bottom row were
considered to be included in bounds that are expressed as an SkIRect.
Companion Chromium CL required for roll: https://codereview.chromium.org/12221077/
TEST=TileGrid unit test
Review URL: https://codereview.appspot.com/7350050
git-svn-id: http://skia.googlecode.com/svn/trunk@7885 2bbb7eff-a529-9590-31e7-b0007b416f81