Simplify GlyphPage initialization for SegmentedFontData

Instead of filling a scratch page and merging it to m_page, just call
fill() on m_page in reverse order of the segmented ranges.
Also change OpenTypeVerticalData::substituteWithVerticalGlyphs() to check
FontData since the page passed may contain different FontData.

BUG=none
TEST=blink_platform_unittests

Review URL: https://codereview.chromium.org/235863018

git-svn-id: svn://svn.chromium.org/blink/trunk@171835 bbb929c8-8fbe-4397-9dbb-9b2b20218538