DM: assert we ran as many tasks as we thought we would

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1731783003

Review URL: https://codereview.chromium.org/1731783003

dm/DM.cpp

@@ -1192,6 +1192,9 @@ int dm_main() {
     parallel.wait();
     gDefinitelyThreadSafeWork.wait();
 
+    // We'd better have run everything.
+    SkASSERT(gPending == 0);
+
     // At this point we're back in single-threaded land.
     sk_tool_utils::release_portable_typefaces();