Remove implicit use of local fonts for families defined via @font-face

This patch removes the logic in CSSSegmentedFontFaceCache that attempts to
substitute web font faces if they are also installed locally.
This hack was introduced in https://bugs.webkit.org/show_bug.cgi?id=18863
and now works only on Mac. Windows no longer uses this since the switch to
skia (r158845).

Removing this hack since this makes @font-faces render differntly when
fonts are installed locally. Also, the CSS Fonts Module Level 3 explicitly
says that this should not happen:
http://www.w3.org/TR/css3-fonts/#font-style-matching
"""
If no faces are present for a family defined via @font-face rules, the
family should be treated as missing; matching a platform font with the
same name must not occur in this case.
"""

BUG=120453

Review URL: https://codereview.chromium.org/77243005

git-svn-id: svn://svn.chromium.org/blink/trunk@162484 bbb929c8-8fbe-4397-9dbb-9b2b20218538