platform/mac/editing/input/devanagari-ligature.html is flaky on Yosemite, ligature...

platform/mac/editing/input/devanagari-ligature.html is flaky on Yosemite, ligature fails to form
https://bugs.webkit.org/show_bug.cgi?id=138683

Reviewed by Darin Adler.

This patch changes how we check fonts for equality. In particular, this patch adds a
objectForEqualityCheck() to Cocoa's FontPlatformData, and callers should pass this object
to CFEqual() to determine if two platform fonts are equal. This patch also migrates all
call sites to using this function.

I don't want to implement operator==() because there are many cases where the same font
is compared against many others, and this solution is cleaner than caching a comparison
object inside the font object itself.

No new tests because this is covered by platform/mac/editing/input/devanagari-ligature.html.

* platform/graphics/FontPlatformData.h:
* platform/graphics/cocoa/FontPlatformDataCocoa.mm:
(WebCore::FontPlatformData::objectForEqualityCheck):
* platform/graphics/mac/ComplexTextControllerCoreText.mm:
(WebCore::ComplexTextController::collectComplexTextRunsForCharacters):
* platform/graphics/mac/GlyphPageTreeNodeMac.cpp:
(WebCore::GlyphPage::fill):
* platform/graphics/mac/SimpleFontDataMac.mm:
(WebCore::SimpleFontData::canRenderCombiningCharacterSequence):
* platform/spi/cocoa/CoreTextSPI.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@177689 268f45cc-cd09-0410-ab3c-d52691b4dbfc