Use SkMutex::assertHeld in SkPDFFont and SkPDFShader.

src/pdf/SkPDFFont.cpp

Index: src/pdf/SkPDFFont.cpp
diff --git a/src/pdf/SkPDFFont.cpp b/src/pdf/SkPDFFont.cpp
index 49d383b204ea1f0c25c089fc6ca17efb58ca9db9..014b328030c4e0baa0eacff09150fe510f583264 100644
--- a/src/pdf/SkPDFFont.cpp
+++ b/src/pdf/SkPDFFont.cpp
@@ -872,15 +872,13 @@ SkPDFFont* SkPDFFont::getFontSubset(const SkPDFGlyphSet*) {
 
 // static
 SkTDArray<SkPDFFont::FontRec>& SkPDFFont::CanonicalFonts() {
-    // This initialization is only thread safe with gcc.
+    SkPDFFont::CanonicalFontsMutex().assertHeld();
     static SkTDArray<FontRec> gCanonicalFonts;
     return gCanonicalFonts;
 }
 
 // static
 SkBaseMutex& SkPDFFont::CanonicalFontsMutex() {
-    // This initialization is only thread safe with gcc, or when
-    // POD-style mutex initialization is used.
     SK_DECLARE_STATIC_MUTEX(gCanonicalFontsMutex);
     return gCanonicalFontsMutex;
 }