Use static_assert instead of SK_COMPILE_ASSERT.

Now that static_assert is allowed, there is no need to use a non-
standard compile time assertion

Review URL: https://codereview.chromium.org/1306443004
diff --git a/src/pdf/SkPDFTypes.h b/src/pdf/SkPDFTypes.h
index 23a30b9..56e534b 100644
--- a/src/pdf/SkPDFTypes.h
+++ b/src/pdf/SkPDFTypes.h
@@ -158,7 +158,7 @@
     SkPDFUnion& operator=(const SkPDFUnion&) = delete;
     SkPDFUnion(const SkPDFUnion&) = delete;
 };
-SK_COMPILE_ASSERT(sizeof(SkString) == sizeof(void*), SkString_size);
+static_assert(sizeof(SkString) == sizeof(void*), "SkString_size");
 
 ////////////////////////////////////////////////////////////////////////////////