Add Segoe UI and its set of linked fonts to the font back list on Windows.

ui/gfx/render_text_harfbuzz.cc

Index: ui/gfx/render_text_harfbuzz.cc
diff --git a/ui/gfx/render_text_harfbuzz.cc b/ui/gfx/render_text_harfbuzz.cc
index a2610f3787288305adb7f9d3c7b07044d8391511..9a9d76c91d2898225ed7ec8b62156f843bc5998c 100644
--- a/ui/gfx/render_text_harfbuzz.cc
+++ b/ui/gfx/render_text_harfbuzz.cc
@@ -1290,6 +1290,16 @@ void RenderTextHarfBuzz::ShapeRun(const base::string16& text,
     fallback_families.insert(fallback_families.end(),
         uniscribe_fallbacks.begin(), uniscribe_fallbacks.end());
   }
+
+  // Add Segoe UI and its associated linked fonts to the fallback font list to
+  // ensure that the fallback list covers the basic cases.
+  // http://crbug.com/467459. On some Windows configurations the default font
+  // could be a raster font like System, which would not give us a reasonable
+  // fallback font list.
+  std::vector<std::string> default_fallback_families =
+      GetFallbackFontFamilies("Segoe UI");

[msw, 2015/03/18 15:53:23]

(1) Why does the bug only occur with DirectWrite? (users claim that
--disable-directwrite-for-ui is a workaround, how does that mitigate the
issue?). (2) Why does/should this change affect GDI? (3) What happens here if
"Segoe UI" isn't installed? (it's not included with Windows XP by default,
afaict...) (4) Shouldn't we avoid duplicate fonts in the fallback list?
(especially if Segoe UI is already the default font, I think this will *double*
the number of shaping passes for runs that need fallback?). (5) Why don't we use
Tahoma or Microsoft Sans Serif (available on XP and additionally support Hebrew
and Thai characters, beyond Latin/Greek/Cyrillic).

I'm dubious that this is the correct change...

[scottmg, 2015/03/18 16:13:04]

It's definitely quite a mess. We did reproduce corrupted characters in GDI too,
so it's possible we're not reproing the same thing that users are seeing, it's
not clear.

Blink does this, which perhaps we need to consider doing:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

I didn't realize duplicating entries in the list was expensive. If there's no
missing glyphs (i.e. Segoe UI is already in the list and helps) then it won't
get here anyway, so maybe it it shouldn't matter in the common case. (?)

Tahoma might be a better choice. We only have reports from Win7 and Win8
currently where Segoe should be available.

[ananta, 2015/03/18 18:41:12]

The current approach of font fallback does not work on Windows XP anyways. We
use linked fonts which are based off this registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows
NT\CurrentVersion\FontLink\SystemLink.

That key does not exist on Windows XP. 

That being said, as scottmg mentioned the bug does occur with GDI as well if the
system font is set to bitmap fonts like System. 

On a related note on a Windows 7 Korean installation the Microsoft Sans Seriff
font was not present in the list of linked fonts. 
Tahoma is there in the linked fonts for Segoe UI.

We do need a better way to achieve font fallback which makes me wonder why don't
we use skia's fallback logic. Would be better than duplicating the same code in
multiple places.

+  fallback_families.insert(fallback_families.end(),
+      default_fallback_families.begin(), default_fallback_families.end());
 #endif
 
   // Try shaping with the fallback fonts.