Fix glyph bounds calculation for complex text

Change Font::floatWidthForComplexText to correctly compute the glyph bounds by
flooring the top/left instead of ceiling them. The same change was made to
simple text in r174013.

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

R=schenney@chromium.org
BUG=372787
TEST=fast/block/lineboxcontain/glyphs.html

Review URL: https://codereview.chromium.org/287733004

git-svn-id: svn://svn.chromium.org/blink/trunk@174026 bbb929c8-8fbe-4397-9dbb-9b2b20218538