SuperSize: Caspian: Display .sizediffs in diff mode

Previously the WebAssembly web worker assumed that it should render in
diff mode if and only if it was passed a |before| to compare against.
With .sizediffs, this assumption no longer holds - the WebAssembly
worker extracts both .size files from a single field, so we have to tell
JavaScript that it's a diff.

Change-Id: I972f7eaca9508c87c033b3f78ad169afa70f2ebc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1954194
Commit-Queue: Jasper Chapman-Black <jaspercb@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722528}

tools/binary_size/libsupersize/caspian/caspian_web.cc

@@ -60,7 +60,10 @@ void LoadBeforeSizeFile(const char* compressed, size_t size) {
   ParseSizeInfo(compressed, size, before_info.get());
 }
 
-void BuildTree(bool method_count_mode,
+// Updates |builder| with provided filters and constructs the new tree.
+// Typically called when the front-end form updates, to apply any new filters.
+// Returns: True if the resulting tree is a diff, false if it is a snapshot.
+bool BuildTree(bool method_count_mode,
                const char* group_by,
                const char* include_regex_str,
                const char* exclude_regex_str,
@@ -158,6 +161,8 @@ void BuildTree(bool method_count_mode,
     exit(1);
   }
   builder->Build(std::move(lens), sep, method_count_mode, filters);
+
+  return bool(diff_info);
 }
 
 const char* Open(const char* path) {

tools/binary_size/libsupersize/static/tree-worker-wasm.js

@@ -128,10 +128,10 @@ async function buildTree(
     }
 
     const BuildTree = Module.cwrap(
-        'BuildTree', 'void',
+        'BuildTree', 'bool',
         ['bool', 'string', 'string', 'string', 'string', 'number', 'number']);
     const start_time = Date.now();
-    BuildTree(
+    const diffMode = BuildTree(
         methodCountMode, groupBy, includeRegex, excludeRegex,
         includeSections, minSymbolSize, flagToFilter);
     console.log(
@@ -141,9 +141,9 @@ async function buildTree(
 
     const root = await Open('');
     return {
-      root: root,
+      root,
       percent: 1.0,
-      diffMode: beforeFetcher !== null,  // diff mode
+      diffMode,
     };
   });
 }