Webview tag creation should be using storage partitions.

content/browser/browser_plugin/browser_plugin_embedder.cc

Index: content/browser/browser_plugin/browser_plugin_embedder.cc
diff --git a/content/browser/browser_plugin/browser_plugin_embedder.cc b/content/browser/browser_plugin/browser_plugin_embedder.cc
index abdde143de2e21e2cbeac70838a506ff710fb414..7e283fe4e2ec92c3a93bda09912d34cb80c8e98f 100644
--- a/content/browser/browser_plugin/browser_plugin_embedder.cc
+++ b/content/browser/browser_plugin/browser_plugin_embedder.cc
@@ -21,6 +21,7 @@
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/web_contents_view.h"
 #include "content/public/common/url_constants.h"
+#include "net/base/escape.h"
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebInputEvent.h"
 #include "ui/gfx/size.h"
 #include "ui/surface/transport_dib.h"
@@ -88,11 +89,30 @@ void BrowserPluginEmbedder::CreateGuest(RenderViewHost* render_view_host,
   BrowserPluginGuest* guest = GetGuestByInstanceID(instance_id);
   CHECK(!guest);
 
+  // Validate that the partition id coming from the renderer is valid UTF-8,
+  // since we depend on this in other parts of the code, such as FilePath
+  // creation.
+  CHECK(IsStringUTF8(storage_partition_id));
+
   const std::string& host =
       render_view_host->GetSiteInstance()->GetSiteURL().host();
+  std::string url_encoded_partition =
+      net::EscapeQueryParamValue(storage_partition_id, false);
+
+  // The SiteInstance of a given webview tag is based on the fact that it's a
+  // guest process in addition to which platform application the tag belongs to
+  // and what storage partition is in use, rather than the URL that the tag is
+  // being navigated to.
+  GURL guest_site(
+      base::StringPrintf("%s://%s/%s?%s", chrome::kGuestScheme,
+                         host.c_str(), persist_storage ? "persist" : "",
+                         url_encoded_partition.c_str()));
+  SiteInstance* guest_site_instance = SiteInstance::CreateForURL(
+      web_contents()->GetBrowserContext(), guest_site);
+
   guest_web_contents = WebContentsImpl::CreateGuest(
       web_contents()->GetBrowserContext(),
-      host,
+      guest_site_instance,
       instance_id,
       focused,
       visible);