SuperSize: Improve .sizediff usability

* Use WebAssembly backend for .sizediff
* Fix bug where an empty .sizediff included all before/after symbols.
* When displaying diff status, handle case where tree node has no
  children (I think this case only occurs when loading an empty diff).

Change-Id: Ibb9aa309dacc87d92f2166922350a235297d85f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1945361Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Jasper Chapman-Black <jaspercb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720569}

tools/binary_size/libsupersize/file_format.py

@@ -178,7 +178,9 @@ def _SaveSizeInfoToFile(size_info,
     file_object: File opened for writing
     sparse_symbols: If present, only save these symbols to the file
   """
-  raw_symbols = sparse_symbols or size_info.raw_symbols
+  raw_symbols = sparse_symbols
+  if raw_symbols is None:
+    raw_symbols = size_info.raw_symbols
   # Created by supersize header
   file_obj.write('# Created by //tools/binary_size\n')
   file_obj.write('%s\n' % _SERIALIZATION_VERSION)

tools/binary_size/libsupersize/static/start-worker.js

@@ -7,7 +7,8 @@
 
 let _innerWorker = null;
 const urlParams = new URLSearchParams(window.location.search);
-if (urlParams.get('load_url').endsWith('.size')) {
+const loadUrl = urlParams.get('load_url');
+if (loadUrl.endsWith('.size') || loadUrl.endsWith('.sizediff')) {
   console.log('Using WebAssembly web worker');
   _innerWorker = new Worker('tree-worker-wasm.js');
 } else {

tools/binary_size/libsupersize/static/state.js

@@ -325,15 +325,17 @@ function _makeIconTemplateGetter() {
    * @returns {SVGSVGElement}
    */
   function getDiffStatusTemplate(node) {
+    const entries = Object.entries(node.childStats);
     let key = 'unchanged';
-    // Leaf nodes only have one stat entry.
-    const statsEntry = Object.entries(node.childStats)[0][1];
-    if (statsEntry.added) {
-      key = 'added';
-    } else if (statsEntry.removed) {
-      key = 'removed';
-    } else if (statsEntry.changed) {
-      key = 'changed';
+    if (entries.length != 0) {
+      const statsEntry = entries[0][1];
+      if (statsEntry.added) {
+        key = 'added';
+      } else if (statsEntry.removed) {
+        key = 'removed';
+      } else if (statsEntry.changed) {
+        key = 'changed';
+      }
     }
     return statusIcons[key].cloneNode(true);
   }