[script-streaming] Fix weak access to loader clent

The ResponseBodyLoaderClient is held weakly by the script streaming
SourceStream class, which means we have to manually check for a null
client when notifying it that we've received data.

Bug: 1098874
Change-Id: I9e2b307a5fcc5fe13f489773a3657f55ba0c1180
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2287310Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786205}

third_party/blink/renderer/bindings/core/v8/script_streamer.cc

@@ -233,6 +233,11 @@ class SourceStream : public v8::ScriptCompiler::ExternalSourceStream {
       ResponseBodyLoaderClient* response_body_loader_client,
       std::unique_ptr<char[]> data,
       uint32_t data_size) {
+    // The response_body_loader_client is held weakly, so it may be dead by the
+    // time this callback is called. If so, we can simply drop this chunk.
+    if (!response_body_loader_client)
+      return;
+
     response_body_loader_client->DidReceiveData(
         base::make_span(data.get(), data_size));
   }