Rename FontPlatformDataCocoa and remove fonts/cocoa directory

Rename the FontPlatformDataCocoa.mm file to FontPlatformDataMac.mm and
move it to the fonts/mac directory where the remaining mac specific font
code resides. Also removes the fonts/cocoa directory as this was the
last file in that directory.

BUG=286450
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/623833002

git-svn-id: svn://svn.chromium.org/blink/trunk@183168 bbb929c8-8fbe-4397-9dbb-9b2b20218538