Allow custom font collection for DirectWrite font manager.

src/ports/SkFontMgr_win_dw.cpp

Index: src/ports/SkFontMgr_win_dw.cpp
diff --git a/src/ports/SkFontMgr_win_dw.cpp b/src/ports/SkFontMgr_win_dw.cpp
index ef09210e38de9669ddb141101ee0a52af9fe4380..11afec3b53623205dad6612ab7cc0057c958181e 100644
--- a/src/ports/SkFontMgr_win_dw.cpp
+++ b/src/ports/SkFontMgr_win_dw.cpp
@@ -1067,7 +1067,8 @@ SkTypeface* SkFontStyleSet_DirectWrite::matchStyle(const SkFontStyle& pattern) {
 ////////////////////////////////////////////////////////////////////////////////
 #include "SkTypeface_win.h"
 
-SK_API SkFontMgr* SkFontMgr_New_DirectWrite(IDWriteFactory* factory) {
+SK_API SkFontMgr* SkFontMgr_New_DirectWrite(IDWriteFactory* factory,
+                                            IDWriteFontCollection* collection) {
     if (nullptr == factory) {
         factory = sk_get_dwrite_factory();
         if (nullptr == factory) {
@@ -1075,9 +1076,12 @@ SK_API SkFontMgr* SkFontMgr_New_DirectWrite(IDWriteFactory* factory) {
         }
     }
 
-    SkTScopedComPtr<IDWriteFontCollection> sysFontCollection;
-    HRNM(factory->GetSystemFontCollection(&sysFontCollection, FALSE),
-         "Could not get system font collection.");
+    SkTScopedComPtr<IDWriteFontCollection> systemFontCollection;
+    if (nullptr == collection) {
+        HRNM(factory->GetSystemFontCollection(&systemFontCollection, FALSE),
+             "Could not get system font collection.");
+        collection = systemFontCollection.get();
+    }
 
     WCHAR localeNameStorage[LOCALE_NAME_MAX_LENGTH];
     WCHAR* localeName = nullptr;
@@ -1095,7 +1099,7 @@ SK_API SkFontMgr* SkFontMgr_New_DirectWrite(IDWriteFactory* factory) {
         };
     }
 
-    return new SkFontMgr_DirectWrite(factory, sysFontCollection.get(), localeName, localeNameLen);
+    return new SkFontMgr_DirectWrite(factory, collection, localeName, localeNameLen);
 }
 
 #include "SkFontMgr_indirect.h"