Fix x-position to character mappings on harfbuzz platforms

The initial value of |currentX| in
HarfBuzzRun::characterIndexForXPosition should be half of the advance
of the first cluster. This change fixes the selection bug in
fast/text/atsui-rtl-override-selection.html

BUG=386909

Review URL: https://codereview.chromium.org/342943009

git-svn-id: svn://svn.chromium.org/blink/trunk@176801 bbb929c8-8fbe-4397-9dbb-9b2b20218538