Output 0 instead of null for the frameTable's innerWindowID column.

The profiler front-end treats null and 0 the same in this column.
Really we don't want to have this column at all, see firefox-devtools/profiler#5006 .

But 0 at least takes up fewer bytes in the JSON file than null.

Author: mstange

fxprof-processed-profile/src/frame_table.rs

@@ -109,7 +109,7 @@ impl Serialize for FrameTable {
         map.serialize_entry("subcategory", &self.subcategories)?;
         map.serialize_entry("func", &self.funcs)?;
         map.serialize_entry("nativeSymbol", &self.native_symbols)?;
-        map.serialize_entry("innerWindowID", &SerializableSingleValueColumn((), len))?;
+        map.serialize_entry("innerWindowID", &SerializableSingleValueColumn(0, len))?;
         map.serialize_entry("line", &SerializableSingleValueColumn((), len))?;
         map.serialize_entry("column", &SerializableSingleValueColumn((), len))?;
         map.end()

fxprof-processed-profile/tests/integration_tests/main.rs

@@ -582,22 +582,22 @@ fn profile_without_js() {
                     2
                   ],
                   "innerWindowID": [
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null,
-                    null
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0
                   ],
                   "line": [
                     null,
@@ -1102,8 +1102,8 @@ fn profile_with_js() {
                     null
                   ],
                   "innerWindowID": [
-                    null,
-                    null
+                    0,
+                    0
                   ],
                   "line": [
                     null,