remove SkWriteBuffer::getWriter32().

SkWriteBuffer exposes its lower-level implementation SkWriter32 through
this one call.  It's not currently used in any interesting way:
   - write_encoded_bitmap() uses it to manually re-create writeDataAsByteArray();
   - unit tests use it incidentally as a quick way to read the serialized bytes.

This should be SkWriteBuffer no longer necessarily needs to have an SkWriter32.

Landing this will let us then remove SkWriter32::contiguousArray().

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1936563002

TBR=reed@google.com
Just deleting API

Review-Url: https://codereview.chromium.org/1936563002
diff --git a/tests/PaintTest.cpp b/tests/PaintTest.cpp
index 253ce86..9865344 100644
--- a/tests/PaintTest.cpp
+++ b/tests/PaintTest.cpp
@@ -258,8 +258,9 @@
     SkWriteBuffer writer;
     paint.flatten(writer);
 
-    const uint32_t* written = writer.getWriter32()->contiguousArray();
-    SkReadBuffer reader(written, writer.bytesWritten());
+    SkAutoMalloc buf(writer.bytesWritten());
+    writer.writeToMemory(buf.get());
+    SkReadBuffer reader(buf.get(), writer.bytesWritten());
 
     SkPaint paint2;
     paint2.unflatten(reader);
@@ -297,7 +298,10 @@
     SkWriteBuffer writer;
     paint.flatten(writer);
 
-    SkReadBuffer reader(writer.getWriter32()->contiguousArray(), writer.bytesWritten());
+    SkAutoMalloc buf(writer.bytesWritten());
+    writer.writeToMemory(buf.get());
+    SkReadBuffer reader(buf.get(), writer.bytesWritten());
+
     SkPaint other;
     other.unflatten(reader);
     ASSERT(reader.offset() == writer.bytesWritten());