Add unit test for GlyphPageTreeNode.

This patch also moves the functionality of GlyphPage::fill() to
SimpleFontData::fillGlyphPage(). This new method is overridden by the
fake SimpleFontData class used in GlyphPageTreeNodeTest.

BUG=none
TEST=blink_platform_unittests

Review URL: https://codereview.chromium.org/240003002

git-svn-id: svn://svn.chromium.org/blink/trunk@171825 bbb929c8-8fbe-4397-9dbb-9b2b20218538