Fix glyph bounds calculation

Change Font::floatWidthForSimpleText to correctly compute the glyph bounds by
flooring the top/left instead of ceiling them. This matches the ascent/decent
logic and ensures accurate metrics.

This doesn't have any visual effect unless subpixel font scaling is enabled.

TEST=fast/block/lineboxcontain/glyphs.html
BUG=372787

Review URL: https://codereview.chromium.org/287633002

git-svn-id: svn://svn.chromium.org/blink/trunk@174013 bbb929c8-8fbe-4397-9dbb-9b2b20218538