Create direct write font proxy classes and unit tests.

content/common/font_warmup_win.h

Index: content/common/font_warmup_win.h
diff --git a/content/common/font_warmup_win.h b/content/common/font_warmup_win.h
index 7a0e9b502c7e8c4c3d5c36d29532c3305d63cf8f..587b731a7b980b555181020d13447ca6466d100e 100644
--- a/content/common/font_warmup_win.h
+++ b/content/common/font_warmup_win.h
@@ -50,6 +50,24 @@ CONTENT_EXPORT void SetPreSandboxWarmupFontMgrForTesting(SkFontMgr* fontmgr);
 // Warmup the direct write font manager for content processes.
 CONTENT_EXPORT void WarmupDirectWrite();
 
+// Directwrite connects to the font cache service to retrieve information about
+// fonts installed on the system etc. This works well outside the sandbox and
+// within the sandbox as long as the lpc connection maintained by the current
+// process with the font cache service remains valid. It appears that there
+// are cases when this connection is dropped after which directwrite is unable
+// to connect to the font cache service which causes problems with characters
+// disappearing.
+// Directwrite has fallback code to enumerate fonts if it is unable to connect
+// to the font cache service. We need to intercept the following APIs to
+// ensure that it does not connect to the font cache service.
+// NtALpcConnectPort
+// OpenSCManagerW
+// OpenServiceW
+// StartServiceW
+// CloseServiceHandle.
+// These are all IAT patched.
+CONTENT_EXPORT void PatchServiceManagerCalls();
+
 }  // namespace content
 
 #endif  // CONTENT_COMMON_FONT_WARMUP_WIN_H_