GlyphPage: Bake per-glyph font data array into same allocation as GlyphPage.

A hopeful fix for REGRESSION(r143125): ~5% performance hit on Chromium's intl2 page cycler
<http://webkit.org/b/108835>

Reviewed by Antti Koivisto.

Rewire GlyphPage so that we have to decide at creation time whether there will be a per-glyph
array of SimpleFontData* or not. This removes one allocation and one step of indirection for
pages with glyphs from mixed fonts.

* platform/graphics/GlyphPage.h:
(WebCore::GlyphPage::createForMixedFontData):
(WebCore::GlyphPage::createForSingleFontData):
(WebCore::GlyphPage::createCopiedSystemFallbackPage):
(WebCore::GlyphPage::~GlyphPage):
(WebCore::GlyphPage::glyphDataForIndex):
(WebCore::GlyphPage::fontDataForCharacter):
(WebCore::GlyphPage::setGlyphDataForIndex):
(WebCore::GlyphPage::removeFontDataFromSystemFallbackPage):
(WebCore::GlyphPage::GlyphPage):
(WebCore::GlyphPage::hasPerGlyphFontData):
(GlyphPage):
* platform/graphics/GlyphPageTreeNode.cpp:
(WebCore::GlyphPageTreeNode::initializePage):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143601 268f45cc-cd09-0410-ab3c-d52691b4dbfc