Using x-advance for emphasis mark drawing.

According to Behdad's suggestions for complex text CSS emphasis
in https://codereview.chromium.org/130433006#msg11 I would like
to change emphasis glyph placement to half of the x-advance, instead
of half the glyph width. This works better when we land the support
for emphasis marks for complex text with FontHarfBuzz.

BUG=335552
TBR=levi,eae,behdad

Review URL: https://codereview.chromium.org/152093006

git-svn-id: svn://svn.chromium.org/blink/trunk@167094 bbb929c8-8fbe-4397-9dbb-9b2b20218538