Use NonBreakingSpace glyph where available

Change GlyphPageTreeNode::initializePage to not override the NBSP glyph
with space as some fonts may render non breaking spaces with a glyph
other than space. As we already have logic falling back on space if the
nbsp glyph is missing from the font no further logic is needed.

R=pdr@chromium.org
BUG=423539
TEST=fonts/cursive.html

Review URL: https://codereview.chromium.org/705163003

git-svn-id: svn://svn.chromium.org/blink/trunk@184938 bbb929c8-8fbe-4397-9dbb-9b2b20218538