Enable HarfBuzz to report used fallback fonts

HarfBuzz maintains information about continuous sequences
of characters supported by the same font. We just need to collect
and pass that information when a caller needs it. This is preparation
for removing the CoreText shaper on Mac, as that one is able
to report this information. Passing up the used fallback fonts
information leads to slight changes in line height, as the layout
code uses all used fonts' metrics information to calculate
the total line height.

BUG=399639
R=eae,behdad

Review URL: https://codereview.chromium.org/418223014

git-svn-id: svn://svn.chromium.org/blink/trunk@181347 bbb929c8-8fbe-4397-9dbb-9b2b20218538