Support user-configurable function to create IDWriteFactory

src/utils/win/SkDWrite.cpp

Index: src/utils/win/SkDWrite.cpp
diff --git a/src/utils/win/SkDWrite.cpp b/src/utils/win/SkDWrite.cpp
index 16e8ddc4d0d93c4103283616199df0ed5df9aa18..156b36f91376f54fffc107eb483bae682474c2a2 100644
--- a/src/utils/win/SkDWrite.cpp
+++ b/src/utils/win/SkDWrite.cpp
@@ -13,8 +13,18 @@
 #include <dwrite.h>
 
 static IDWriteFactory* gDWriteFactory = NULL;
+static void (*gDWriteFactoryProc)(IDWriteFactory**) = NULL;
+
+void SkSetDirectWriteFactoryProc(void (*proc)(IDWriteFactory**)) {
+    gDWriteFactoryProc = proc;
+}
 
 static void create_dwrite_factory(IDWriteFactory** factory) {
+    if (gDWriteFactoryProc) {
+        gDWriteFactoryProc(factory);
+        return;
+    }
+
     typedef decltype(DWriteCreateFactory)* DWriteCreateFactoryProc;
     DWriteCreateFactoryProc dWriteCreateFactoryProc = reinterpret_cast<DWriteCreateFactoryProc>(
         GetProcAddress(LoadLibraryW(L"dwrite.dll"), "DWriteCreateFactory"));