[iOS] Support Khmer and Lao fallback fonts

[iOS] Support Khmer and Lao fallback fonts
https://bugs.webkit.org/show_bug.cgi?id=134114

Reviewed by Enrica Casucci.

Source/WebCore:
iOS has fonts which have glyphs for the Khmer and Lao languages.

This patch also does a little refactoring of the language-specific fallback
font path to increase readability.

Test: fast/text/khmer-lao-font.html

* platform/graphics/ios/FontCacheIOS.mm:
(WebCore::languageSpecificFallbackFont):
(WebCore::FontCache::systemFallbackForCharacters):

LayoutTests:
Simply dump the render tree of a page that includes Khmer and Lao text. The success
of this test depends on font font metrics being different than system fallback font
(which just shows rectangles instead of glyphs).

* fast/text/khmer-lao-font.html: Added.
* platform/ios-sim/fast/text/khmer-lao-font-expected.txt: Added.
* platform/mac/fast/text/khmer-lao-font-expected.txt: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170207 268f45cc-cd09-0410-ab3c-d52691b4dbfc