No one uses SkPicture::swap.

tests/PictureTest.cpp

Index: tests/PictureTest.cpp
diff --git a/tests/PictureTest.cpp b/tests/PictureTest.cpp
index 6969524bd7156d049cf7dd611ac2d8de86de541e..c554a1223ebacc2cd909e42822665548de63cbc4 100644
--- a/tests/PictureTest.cpp
+++ b/tests/PictureTest.cpp
@@ -1561,16 +1561,6 @@ static void test_gen_id(skiatest::Reporter* reporter) {
     SkPicture emptyCopy(empty);
     REPORTER_ASSERT(reporter, empty.uniqueID() != emptyCopy.uniqueID());
 
-    // test out swap
-    {
-        SkPicture swapWithData;
-        uint32_t beforeID1 = swapWithData.uniqueID();
-        uint32_t beforeID2 = copyWithData.uniqueID();
-        swapWithData.swap(copyWithData);
-        REPORTER_ASSERT(reporter, copyWithData.uniqueID() == beforeID1);
-        REPORTER_ASSERT(reporter, swapWithData.uniqueID() == beforeID2);
-    }
-
     // test out clone
     {
         SkAutoTUnref<SkPicture> cloneWithData(hasData->clone());