Add logic to switch to DirectWrite font proxy via field trial.

content/public/common/dwrite_font_platform_win.h

Index: content/public/common/dwrite_font_platform_win.h
diff --git a/content/public/common/dwrite_font_platform_win.h b/content/public/common/dwrite_font_platform_win.h
index 4c988bc82eb20acc970f6eba10bcd2547fc4f7e0..d049f280b1cfed4fc0ed3fcea74c9c87e7d81cfe 100644
--- a/content/public/common/dwrite_font_platform_win.h
+++ b/content/public/common/dwrite_font_platform_win.h
@@ -36,6 +36,12 @@ CONTENT_EXPORT bool LoadFontCache(const base::FilePath& path);
 // Added in header mainly for unittest
 CONTENT_EXPORT bool ValidateFontCacheFile(base::File* file);
 
+// Determines whether code should use the DirectWrite font proxy codepath.
+// Note that this function only checks whether the field trial is enabled.
+// Callers are still expected to take appropriate action based on the return
+// value.
+CONTENT_EXPORT bool ShouldUseDirectWriteFontProxyFieldTrial();
+
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_COMMON_DWRITE_FONT_PLATFORM_WIN_H_