Allow webview guests to skip WAR checks in ShouldAllowOpenURL.

chrome/browser/extensions/chrome_content_browser_client_extensions_part.cc

Index: chrome/browser/extensions/chrome_content_browser_client_extensions_part.cc
diff --git a/chrome/browser/extensions/chrome_content_browser_client_extensions_part.cc b/chrome/browser/extensions/chrome_content_browser_client_extensions_part.cc
index 5a8444518041e5bdfee2854215d424deada74763..1a5845da9260813a4b0e0c6bde2473cb6d19cd01 100644
--- a/chrome/browser/extensions/chrome_content_browser_client_extensions_part.cc
+++ b/chrome/browser/extensions/chrome_content_browser_client_extensions_part.cc
@@ -613,6 +613,15 @@ bool ChromeContentBrowserClientExtensionsPart::ShouldAllowOpenURL(
     return true;
   }
 
+  // <webview> guests should be allowed to load only webview-accessible
+  // resources, but that check is done later in
+  // AllowCrossRendererResourceLoadHelper, so allow <webview> guests to proceed

[Devlin, 2017/03/22 21:58:56]

This makes me a bit sad... what would need to happen to combine these checks? 
(I don't trust my knowledge of when each is called.)

[alexmos, 2017/03/22 23:56:39]

I think this one is called on the UI thread only for navigations, before the
request is kicked off, but only on the OpenURL/transfer path. 
AllowCrossRendererResourceLoad happens on the IO thread for all network
requests, including subresources.  So they're not exactly the same, and I think
we had issues doing some of those checks on the IO thread due to not having some
necessary info there.  One thing that might help is PlzNavigate, as (1) it will
remove the transfer path, so we won't need to worry about this for transfers
anymore, and (2) it already reuses AllowCrossRendererResourceLoadHelper for main
frame requests, which it calls from the UI thread in
ExtensionNavigationThrottle.  I think we could at least combine the checks here
with those in ExtensionNavigationThrottle.  I definitely agree we have too many
places enforcing these things, so this is something we need to think about for
sure.

+  // here and rely on that check instead.  See https://crbug.com/691941.
+  if (site_url.SchemeIs(content::kGuestScheme)) {
+    *result = true;
+    return true;
+  }
+
   if (WebAccessibleResourcesInfo::IsResourceWebAccessible(to_extension,
                                                           to_url.path())) {
     *result = true;