add Make factory to SkMemoryStream (simplify call-sites)

Bug: skia:6888
Change-Id: Ia4e432673ed089a91229697c8dde0489f220000d
Reviewed-on: https://skia-review.googlesource.com/26884
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/tests/CodecTest.cpp b/tests/CodecTest.cpp
index 698af46..4e68cd1 100644
--- a/tests/CodecTest.cpp
+++ b/tests/CodecTest.cpp
@@ -1389,8 +1389,8 @@
 
 static void test_invalid_header(skiatest::Reporter* r, const char* path) {
     SkString resourcePath = GetResourcePath(path);
-    std::unique_ptr<SkFILEStream> stream(new SkFILEStream(resourcePath.c_str()));
-    if (!stream->isValid()) {
+    auto stream = SkFILEStream::Make(resourcePath.c_str());
+    if (!stream) {
         return;
     }