[sheriff] add scoped trace to LoadAllPdfsTests

There's a leak being detected by the Chromium OS ASan LSan bot
when loading the test pdf files. Adding tracing of the filenames
to see which one(s).

Bug: 826055
Change-Id: I9ad1c6a3d67c906e2713492271b68139803d59c4
R: battre@chromium.org
TBR: sky@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/980962Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546147}

chrome/browser/pdf/pdf_extension_test.cc

@@ -242,6 +242,7 @@ class PDFExtensionTest : public ExtensionApiTest,
       ASSERT_FALSE(filename.empty());
 
       std::string pdf_file = dir_name + "/" + filename;
+      SCOPED_TRACE(pdf_file);
       if (static_cast<int>(base::Hash(filename) % kNumberLoadTestParts) == k) {
         LOG(INFO) << "Loading: " << pdf_file;
         bool success = LoadPdf(embedded_test_server()->GetURL("/" + pdf_file));