Handle multiple fragments in the getCharNumberAtPosition SVGTextQuery

The start position (and thus indirectly end position) used to compute the
extents of a glyph to check against the point is computed based on the
'processedCharacter' query data state. Said state is only updated after
each text box has been processed, meaning that for a text box with
multiple fragments, the offset of the fragment within the box needs to be
included to get the correct start/end position.

BUG=387725

Review URL: https://codereview.chromium.org/349573005

git-svn-id: svn://svn.chromium.org/blink/trunk@176936 bbb929c8-8fbe-4397-9dbb-9b2b20218538