Consolidate SkStream copying methods

Make SkCopyStreamToData call SkStreamCopy, removing duplicate code.

The former still has its own method of copying with a length, since
it saves one copy.

BUG=skia:4788
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1640793002

Review URL: https://codereview.chromium.org/1640793002

src/core/SkStream.cpp

@@ -890,12 +890,9 @@ SkData* SkCopyStreamToData(SkStream* stream) {
     }
 
     SkDynamicMemoryWStream tempStream;
-    const size_t bufferSize = 4096;
-    char buffer[bufferSize];
-    do {
-        size_t bytesRead = stream->read(buffer, bufferSize);
-        tempStream.write(buffer, bytesRead);
-    } while (!stream->isAtEnd());
+    if (!SkStreamCopy(&tempStream, stream)) {
+        return nullptr;
+    }
     return tempStream.copyToData();
 }