Enable "system-ui" generic font family

third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm

Index: third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
diff --git a/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm b/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
index 5956c7a737beead71d6df78f75f6246d807c35e4..a9b9a9eccb1958e2dd3d24e0dc0f1b50ab24f766 100644
--- a/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
+++ b/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
@@ -61,6 +61,19 @@ namespace blink {
 
 const char* kColorEmojiFontMac = "Apple Color Emoji";
 
+// static
+const AtomicString& FontCache::legacySystemFontFamily() {
+  DEFINE_STATIC_LOCAL(AtomicString, legacySystemFontFamily,
+                      ("BlinkMacSystemFont"));
+  return legacySystemFontFamily;
+}
+
+// static
+const AtomicString& FontCache::systemFontFamily(float) {

[drott, 2016/10/12 11:01:59]

When will you need the parameter? If Mac instantiates with "systemFontForSize"
in FontFamilyMatcherMac, do we need this parameter here?

[kojii, 2016/10/12 11:14:52]

I'm thinking to replace "system-ui" with ".SF NS Text" or ".SF NS Display", then
with the list of cascading list. By replacing family name in this stage, we get
different FontData for each actual fonts.

This API surface supports only the first phase. I believe it's the significant
part of broken letter spacing, and would like to consider the design when I'm
more in sync with your plan to deprecate FontFallbackList and SimpleFontData.

I haven't really well tested replacing ".SF NS Text"/".SF NS Display" works well
yet, should have it soon. That said, this may need a revise, or two. I just
added size now because that gives me where FontCache can replace the family name
(i.e., only after it computed the size and know the locale.)

[drott, 2016/10/13 07:59:34]

So you're saying you would like to move the replacement/resolution of system-ui
from MatchNSFontFamily to here in order for use to get different
FontPlatformData objects?

[kojii, 2016/10/13 09:03:20]

Yes, that way we can keep one SkTypeface/SimpleFontData/hb to map to single
OpenType font. It's not working really well yet, I need more your help on this
(will send separate e-mail) but I hope the direction is right?

[drott, 2016/10/13 12:36:13]

Yes, sounds reasonable.

+  // TODO(kojii): Get the actual font name for the size.
+  return FontFamilyNames::system_ui;
+}
+
 static void invalidateFontCache() {
   if (!isMainThread()) {
     Platform::current()->mainThread()->getWebTaskRunner()->postTask(