Add text-rendering and font-smoothing to the font cache key.

The text-rendering enumeration is added to the key on all platforms
as it exists on all platforms. The font-smoothing enumeration is only
added on Mac as it is CoreGraphics specific. This change fixes several
odd issues with the use of font-smoothing on Mac where it sometimes
did not work, and other times appeared to leak.

This also cleans up the font cache key code a bit.

R=eae@chromium.org

Review URL: https://codereview.chromium.org/26919002

git-svn-id: svn://svn.chromium.org/blink/trunk@159494 bbb929c8-8fbe-4397-9dbb-9b2b20218538