SimpleFontData::fillGlyphPage should not set zero for missing glyphs

From r171835, we call fillGlyphPage() multiple times on a glyph page
when initializing a page for SegmentedFontData. It introduced a bug
where glyph fallback for overlapping unicode ranges does not work
correctly, because fillGlyphPage() may overwrite zero for missing glyphs.

Since GlyphPage's constructor does zero-clear, there is no need to
set zero for missing glyphs.

BUG=none
TEST=fast/css/font-face-unicode-range.html,svg/custom/svg-fonts-segmented.xhtml

Review URL: https://codereview.chromium.org/244253003

git-svn-id: svn://svn.chromium.org/blink/trunk@172107 bbb929c8-8fbe-4397-9dbb-9b2b20218538