Cleaner external buffer handling in SkWriter32

src/core/SkWriter32.cpp

Index: src/core/SkWriter32.cpp
diff --git a/src/core/SkWriter32.cpp b/src/core/SkWriter32.cpp
index 5e89ed655b70e5d83da4df2588704689898b52e6..07a43d782123b8bfb70ec34fb2a48f8a437eea01 100644
--- a/src/core/SkWriter32.cpp
+++ b/src/core/SkWriter32.cpp
@@ -66,3 +66,18 @@ size_t SkWriter32::WriteStringSize(const char* str, size_t len) {
     // add 1 since we also write a terminating 0
     return SkAlign4(lenBytes + len + 1);
 }
+
+void SkWriter32::grow(size_t size) {
+    bool wasExternal = (fExternal != NULL) && (fData == fExternal);
+    // cause the buffer to grow
+    fInternal.setCount(size);
+    fData = fInternal.begin();
+    if (wasExternal) {
+        // we were external, so copy in the data
+        memcpy(fData, fExternal, fUsed);
+    }
+    // read the grown capacity
+    fCapacity = fInternal.reserved();
+    // expand the buffer to it's full capacity
+    fInternal.setCount(fCapacity);
+}