When tracking regions, treat sub-pixel areas as belonging to the region.

It is critical to display list canvases, due to the fact, that we fallback from
rendering on impl thread, when the canvas is not fully redrawn inside each
frame. On Browsermark 2 it is actually messed up due to floating point rounding.
The area in SkScalar is {0.00061, 0.00061, 1599.99997, 999.99998}, rounded down
to {1,1,1599,999} and the entire canvas area seems to be not redrawn.

BUG=386601

Review URL: https://codereview.chromium.org/666253003

git-svn-id: svn://svn.chromium.org/blink/trunk@184327 bbb929c8-8fbe-4397-9dbb-9b2b20218538