Use SkMutex::assertHeld in SkPDFFont and SkPDFShader.

src/pdf/SkPDFShader.cpp

Index: src/pdf/SkPDFShader.cpp
diff --git a/src/pdf/SkPDFShader.cpp b/src/pdf/SkPDFShader.cpp
index 62adf5811db5a3672f73f5e1a8796b6c2596b779..23bd203f9d064cb6ecb069cfff220e15d42b5768 100644
--- a/src/pdf/SkPDFShader.cpp
+++ b/src/pdf/SkPDFShader.cpp
@@ -659,22 +659,20 @@ SkPDFObject* SkPDFShader::GetPDFShader(const SkShader& shader,
 
 // static
 SkTDArray<SkPDFShader::ShaderCanonicalEntry>& SkPDFShader::CanonicalShaders() {
-    // This initialization is only thread safe with gcc.
+    SkPDFShader::CanonicalShadersMutex().assertHeld();
     static SkTDArray<ShaderCanonicalEntry> gCanonicalShaders;
     return gCanonicalShaders;
 }
 
 // static
 SkBaseMutex& SkPDFShader::CanonicalShadersMutex() {
-    // This initialization is only thread safe with gcc or when
-    // POD-style mutex initialization is used.
     SK_DECLARE_STATIC_MUTEX(gCanonicalShadersMutex);
     return gCanonicalShadersMutex;
 }
 
 // static
 SkPDFObject* SkPDFFunctionShader::RangeObject() {
-    // This initialization is only thread safe with gcc.
+    SkPDFShader::CanonicalShadersMutex().assertHeld();
     static SkPDFArray* range = NULL;
     // This method is only used with CanonicalShadersMutex, so it's safe to
     // populate domain.