Add support for 'start_url' in Manifest.

content/browser/manifest/manifest_manager_host.cc

Index: content/browser/manifest/manifest_manager_host.cc
diff --git a/content/browser/manifest/manifest_manager_host.cc b/content/browser/manifest/manifest_manager_host.cc
index d1bf2911272a0e9840695c92ade0ca64f9ce2512..796d0a969379322babb810446cda4ce1a7567006 100644
--- a/content/browser/manifest/manifest_manager_host.cc
+++ b/content/browser/manifest/manifest_manager_host.cc
@@ -109,6 +109,8 @@ void ManifestManagerHost::OnRequestManifestResponse(
   manifest.short_name = base::NullableString16(
         manifest.short_name.string().substr(0, Manifest::kMaxIPCStringLength),
         manifest.short_name.is_null());
+  if (!manifest.start_url.is_empty() && !manifest.start_url.is_valid())
+    manifest.start_url = GURL();
 
   callback->Run(manifest);
   callbacks->Remove(request_id);