PPAPI: Add NetworkIsolationKeys to proxy lookup requests.

This is needed to correctly isolate any DNS requests made by PPAPI in
the context of one page from those made in the context of other pages.

This is a reland of a CL that has been reverted 3 times due to flaky
tests on the bots. Unable to figure out what's going on with the tests,
and unable to repro locally, I've finally given up on them. The way
they were failing is likely due to DNS cache issues, rather than due to
the code itself being flaky.

Bug: 1021661
Change-Id: I261e9890198586ede4ad71377800206dd1a7eedc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2199682Reviewed-by: Eric Orth <ericorth@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769595}

content/browser/renderer_host/pepper/pepper_network_proxy_host.cc

@@ -46,9 +46,9 @@ bool LookUpProxyForURLCallback(
   StoragePartition* storage_partition = BrowserContext::GetStoragePartition(
       site_instance->GetBrowserContext(), site_instance);
 
-  // TODO(https://crbug.com/1021661): Pass in a non-empty NetworkIsolationKey.
   storage_partition->GetNetworkContext()->LookUpProxyForURL(
-      url, net::NetworkIsolationKey::Todo(), std::move(proxy_lookup_client));
+      url, render_frame_host->GetNetworkIsolationKey(),
+      std::move(proxy_lookup_client));
   return true;
 }