Carry advances between runs in HarfBuzzShaper.cpp

When the setDrawRange method is used to restrict painting
to a range, the initial advance has to be carried over to
the first glyph that's being added to the glyph buffer,
otherwise things like selection painting will fail.

Fixes a regression after refactoring GlyphBuffer in r182991.

BUG=420614
R=eae,behdad,leviw

Review URL: https://codereview.chromium.org/627273003

git-svn-id: svn://svn.chromium.org/blink/trunk@183347 bbb929c8-8fbe-4397-9dbb-9b2b20218538