[PDF] Add debug assert to detect failure of GDI cache hack.

If we start hitting this check, then it's likely that something has regressed in WebKit.

Original CL from arthurhsu@chromium.org: http://codereview.appspot.com/5322063/

Review URL: http://codereview.appspot.com/5320062

git-svn-id: http://skia.googlecode.com/svn/trunk@2581 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/pdf/SkPDFFont.cpp b/src/pdf/SkPDFFont.cpp
index e6a0f7f..0da5698 100644
--- a/src/pdf/SkPDFFont.cpp
+++ b/src/pdf/SkPDFFont.cpp
@@ -774,6 +774,7 @@
         fontMetrics =
             SkFontHost::GetAdvancedTypefaceMetrics(fontID, info, NULL, 0);
 #if defined (SK_SFNTLY_SUBSETTER)
+        SkASSERT(fontMetrics);
         SkSafeUnref(fontMetrics.get());  // SkRefPtr and Get both took a ref.
         if (fontMetrics &&
             fontMetrics->fType != SkAdvancedTypefaceMetrics::kTrueType_Font) {