Make a flipped fDirtyBit always mean "this field is not the default".

tests/PaintTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBlurMask.h"
 #include "SkBlurMaskFilter.h"
 #include "SkLayerDrawLooper.h"
@@ skipping 300 matching lines @@
     // test that the rect was reset
     paint.measureText("", 0, &r);
     REPORTER_ASSERT(reporter, r.isEmpty());
 }
 
 #define ASSERT(expr) REPORTER_ASSERT(r, expr)
 
 DEF_TEST(Paint_FlatteningTraits, r) {
     SkPaint paint;
     paint.setColor(0x00AABBCC);
-    paint.setTextScaleX(1.0f);  // Encoded despite being the default value.
+    paint.setTextScaleX(1.0f);  // Default value, ignored.
     paint.setTextSize(19);
     paint.setXfermode(SkXfermode::Create(SkXfermode::kModulate_Mode))->unref();
-    paint.setLooper(NULL);  // Ignored.
+    paint.setLooper(NULL);  // Default value, ignored.
 
     SkWriteBuffer writer;
     SkPaint::FlatteningTraits::Flatten(writer, paint);
-    const size_t expectedBytesWritten = sizeof(void*) == 8 ? 36 : 32;
-    ASSERT(expectedBytesWritten == writer.bytesWritten());
 
-    const uint32_t* written = writer.getWriter32()->contiguousArray();
-    SkASSERT(written != NULL);
-    ASSERT(*written == ((1<<0) | (1<<1) | (1<<2) | (1<<8)));  // Dirty bits for our 4.
+    // BEGIN white box asserts: if the impl changes, these asserts may change
+        const size_t expectedBytesWritten = sizeof(void*) == 8 ? 32 : 28;
+        ASSERT(expectedBytesWritten == writer.bytesWritten());
+
+        const uint32_t* written = writer.getWriter32()->contiguousArray();
+        SkASSERT(written != NULL);
+        ASSERT(*written == ((1<<0) | (1<<1) | (1<<8)));  // Dirty bits for our 3.
+    // END white box asserts
 
     SkReadBuffer reader(written, writer.bytesWritten());
     SkPaint other;
     SkPaint::FlatteningTraits::Unflatten(reader, &other);
     ASSERT(reader.offset() == writer.bytesWritten());
 
     // No matter the encoding, these must always hold.
     ASSERT(other.getColor()      == paint.getColor());
     ASSERT(other.getTextScaleX() == paint.getTextScaleX());
     ASSERT(other.getTextSize()   == paint.getTextSize());
     ASSERT(other.getLooper()     == paint.getLooper());
 
     // We have to be a little looser and compare just the modes.  Pointers might not be the same.
     SkXfermode::Mode otherMode, paintMode;
     ASSERT(other.getXfermode()->asMode(&otherMode));
     ASSERT(paint.getXfermode()->asMode(&paintMode));
     ASSERT(otherMode == paintMode);
 }