Add a leak sanitizer suppression for blink::ResourceFetcher::garbageCollectDocumentResources

In content_browsertests, LSAN reports the leak happens in
ResourceFetcher, but its (persistent|ref) pointer is held
by a live instance of Document or DocumentLoader at the
end of the tests.

BUG=506433

Review URL: https://codereview.chromium.org/1216963014

Cr-Original-Commit-Position: refs/heads/master@{#337371}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2af3041652d3a473995babb42e0d71778427dfdf

sanitizers/lsan_suppressions.cc

@@ -97,6 +97,9 @@ char kLSanDefaultSuppressions[] =
 // http://crbug.com/356306
 "leak:content::SetProcessTitleFromCommandLine\n"
 
+// http://crbug.com/506433
+"leak:blink::ResourceFetcher::garbageCollectDocumentResources\n"
+
 // PLEASE READ ABOVE BEFORE ADDING NEW SUPPRESSIONS.
 
 // End of suppressions.